Kaplan Micro Flashcards
- An unvaccinated dental student has a needle-stick accident involving an empty syringe that
had been previously used on a patient with a known hepatitis B infection. Which of the
following is the most probable outcome for the medical worker?
A. Acute hepatitis followed by recovery
B. “Healthy” carrier
C. Persistent infection followed by recovery
D. Persistent infection progressing to chronic hepatitis
E. Subclinical disease followed by recovery
The correct answer is E. Hepatitis B infection can produce a wide variety of clinical outcomes. The
most common outcome (60% to 65%), however, turns out to be subclinical disease followed by
complete recovery. The other choices listed show other possible outcomes, and their statistical
impact. Approximately 20% to 25% of infected persons develop acute hepatitis (choice A), which is
followed in 99% of these cases by recovery and in about 1% of cases by fulminant hepatitis.
Approximately 5% to 10% of cases become “healthy” carriers (choice B). Approximately 4% of cases
develop persistent infection, 67% to 90% of which then recover (choice C) and 10% to 33% of which
have chronic hepatitis (choice D).
- Biopsy of an ulcerated gastric lesion on a 60-year-old smoker demonstrates the presence of
a gastric carcinoma. If the patient noted that he had severe “heart burn” for the past several
years, with which of the following infectious agents has this type of lesion been most strongly
associated?
A. Epstein-Barr virus
B. Helicobacter pylori
C. Human papilloma virus
D. Molluscum contagiosum virus
E. Schistosoma haematobium
The correct answer is B. The patient has gastric carcinoma, which has been strongly linked, in at
least some studies, to prior gastric infection with Helicobacter pylori. H. pylori has also been
implicated in the etiologies of gastric peptic ulcer, chronic gastritis, and (questionably) gastric
lymphoma. It is believed that treatment of the H. pylori infection with a combination antibiotic
treatment regimen, such as amoxicillin + metronidazole + proton pump inhibitor + bismuth
subsalicylate will decrease the incidence of this type of carcinoma. Epstein-Barr virus (choice A) has
been linked to African Burkitt lymphoma and nasopharyngeal carcinoma. Human papilloma virus
(choice C) has been linked to a variety of warts, condyloma, and genital cancers. Molluscum
contagiosum virus (choice D) is a poxvirus that causes small tumor-like papules of the skin.
Schistosoma haematobium(choice E) has been linked to bladder cancer.
- The biological attribute of influenza A virus, which allows the sudden appearance of
dramatically new genetic variants, is also present in a limited number of other viral families.
Which of the following viruses also possesses this biological attribute?
A. Coronavirus
B. HIV
C. Measles virus
D. Rotavirus
E. Rubella virus
The correct answer is D. Pandemics of influenza A can be caused by the ability of the virus to
undergo dramatic genetic changes of type by reassortment of its segmented RNA genome, a trait
called genetic shift. The only virus on the list that possesses a segmented genome is the rotavirus, in
the reovirus family, which possesses 10-11 segments in its genome. Coronavirus (choice A) is not
segmented and is a cause of the common cold. HIV (choice B) is not segmented and is known for its
genetic drift (minor mutational changes over time caused by an error-prone polymerase), not genetic
shift. Measles virus (choice C) is not segmented and is controlled largely by vaccination. The virus is
known as a paramyxovirus. Rubella virus (choice E) is not segmented. The togavirus causes
arthralgia (primarily in young women), fever, malaise, coryza, lymphadenopathy, and a fine
maculopapular rash.
- Three months after a needle-stick exposure to blood from a patient with hepatitis
1
B, a nurse is evaluated for infection with the virus. Laboratory results reveal: HBsAg absent anti-HBs
antibody absent IgM anti-HBc present IgG anti-HBc absent HBeAg absent. On the basis of these results, which of the following most accurately describes the nurse’s
hepatitis B status?
A. She had been effectively vaccinated against hepatitis B bfore the needle-stick exposure occurred
B. She has mounted an inappropriate antibody response to hepatitis B as a result of an
immunocompromised state
C. She is a carrier of hepatitis B
D. She is actively infected with hepatitis B
E. She was not infected with hepatitis B
The correct answer is choice D. The nurse’s elevated IgM anti-HBc indicates that she was infected
with hepatitis B. Formerly, HBsAg (surface antigen) and anti-HBsAg (antibody to surface antigen)
were used exclusively to determine this. Typically, HBsAg is positive for up to 6 months, and anti
HBsAg is positive for years after that. Unfortunately, this simple scheme has the disadvantage that
many patients have a 2-week to 4-month “window” period, when the surface antigen (HBsAg) and the
antibody (anti-HBs) are not detectable. Presumably, for a relatively brief period, HBsAg production
exactly matches antibody production, and the two coprecipitate such that neither free species is
present in adequate concentration to be detectable. This problem can be circumvented by concurrent
measurements of other antigens and antibodies, including HBeAg, anti HBe, and anti-HBc (HBcAg is
not reliable). During the window period, IgM anti-HBc may be the only marker of recent HBV infection,
as it is in this nurse.
If the nurse had been effectively vaccinated for hepatitis B (choice A), she would have had an
elevated anti-HBs antibody level and no HBsAg present in the serum. Anti-HBc antibody would have
been absent as well. The antibody response to hepatitis B infection was appropriate in this person,
which argues against immunocompromise (choice B). Carriers (choice C) have elevated anti-HBs
and may have persistently elevated HBsAg (in approximately 10% of cases). IgG anti-HBc (not IgM)
predominates in these chronic patients. Had she not been infected with hepatitis B at all (choice E),
IgM anti-HBc would be absent.
- An important factor in determining the likelihood of converting to HIV+ status after a
needlestick injury is which of the following?
A. Depth of the needlestick
B. Volume of blood in the needle
C. Type of needle
D. Health status of source patient
E. All of the above
The correct answer is E. If a healthcare worker is accidently stuck with an HIV-infected bloody
sharp, the worker may or may not contract HIV disease. Conversion following needlestick accidents is
low overall, sometimes estimated at 0.3% (3 per thousand). Several factors, however, have been
identified that make it MORE likely that the accident will transfer the virus to the worker. They include
a large volume of blood, a hollow-bore needle, visible blood on the needle, a deep (not superficial)
stick into deep skin or muscle, and a source patient in late stages of AIDS. Followup of needlestick
injuries is a rapidly evolving field, so be sure to use your most recent notes and test reviews to stay on
top of this subject.
- A 16-year-old boy with sickle cell disease is hospitalized for a severe infection. His
symptoms include fever, chills, cough, and chest pain. Bacteria from the patient’s sputum
yield optochin-sensitive organisms with a positive Quellung reaction. Which of the following is
the most likely pathogen?
A. Escherichia coli
B. Haemophilus influenzae
C. Klebsiella pneumoniae
D. Neisseria gonorrhoeae
E. Streptococcus pneumoniae
The correct answer is E. The combination of optochin sensitivity and positive Quellung reaction is
characteristic of a single organism, Streptococcus pneumoniae (diplococcus). The Quellung reaction
is a capsular swelling caused by contact with specific capsular antisera. The other encapsulated
organisms that have Quellung-positive reactions are Haemophilus influenzae (choice B), Neisseria
meningitidis, and Klebsiella pneumoniae (choice C). None of these organisms, however, are optochin
sensitive. The other choices, Escherichia coli (choice A) and Neisseria gonorrhoeae (choice D), are
not encapsulated.
- A 33-year-old woman has recently returned from overseas and presents with severe, acute,
right upper quadrant abdominal pain, bloody diarrhea, and tenesmus. CT scan of the liver
demonstrates lesions identified as abscesses. Which of the following organisms is the most
likely cause of her illness?
A. Ascaris lumbricoides
B. Entamoeba histolytica
C. Enterobius vermicularis
D. Salmonella typhi
E. Shigella species
The correct answer is B. The patient probably has hepatic amebiasis, a life-threatening complication
of intestinal infection with Entamoeba histolytica. E. histolytica is transmitted by way of the fecal-oral
route. Intestinal colonization (which may be asymptomatic) always precedes infection of the liver. The
abscesses generally contain necrotic debris, with amoebae located along the edges of the abscess.
Patients often present initially with frequent bloody, small-volume stools, often associated with fever,
abdominal cramps, tenesmus, and fecal urgency. When untreated, this condition can result in the
development of hepatic amebiasis. Ascaris lumbricoides (choice A) can cause intestinal obstruction,
but does not usually cause hepatic abscesses. Enterobius vermicularis (choice C) is the pinworm,
which inhabits the rectum. Bloody diarrhea can also be seen with some strains of Salmonella (choice
D) and Shigella (choice E), but these organisms do not usually cause hepatic abscesses. Bacterial
causes of hepatic abscesses include E. coli, Klebsiella, Streptococcus, Staphylococcus, Bacteroides,
and Pseudomonas.
- A 65-year-old man presents with fever, severe headache, and nuchal rigidity. Lumbar
puncture reveals cloudy cerebrospinal fluid (CSF) with elevated neutophils, elevated protein,
and decreased glucose. Which of the following is the most probable etiologic agent of this
condition in a patient of this age?
A. Arbovirus
B. Herpesvirus
C. Mycobacterium tuberculosis
D. Streptococcus pneumoniae
E. Neisseria meningitidis
The correct answer is D. The clinical manifestations (fever, headache, and nuchal rigidity), along
with the CSF findings (increased neutrophils, elevated protein, and reduced glucose), strongly
indicate acute pyogenic (bacterial) meningitis as the underlying condition. Of the microorganisms
listed, either Neisseria meningitidis or Streptococcus pneumoniae can cause this form of meningitis;
however, Streptococcus pneumoniae is by far the most frequent organism causing acute meningitis in
elderly patients. Arboviruses and herpesviruses (choices A and B) can cause an encephalitis
characterized by lymphocytic infiltration of the brain parenchyma and leptomeninges. In this case,
CSF findings would include an increased number of lymphocytes and a normal glucose concentration,
although the protein level in CSF would be increased. Mycobacterium tuberculosis(choice C) may
cause a chronic meningoencephalitis, with a prolonged clinical course. It is characterized
pathologically by a dense granulomatous infiltrate of the base of the brain. Associated CSF findings
include increased lymphocytes and normal or slightly decreased glucose.
- A child has a history of recurrent infections with organisms having polysaccharide antigens
(i.e., Streptococcus pneumoniae and Haemophilus influenzae). This susceptibility can be
explained by a deficiency of
A. C3 nephritic factor
B. C5
C. IgG subclass 2
D. secretory IgA
The correct answer is C. IgG is the predominant antibody in the secondary immune response. IgG
subclass 2 is directed against polysaccharide antigens and is involved in the host defense against
encapsulated bacteria. C3 nephritic factor (choice A) is an IgG autoantibody that binds to C3
convertase, making it resistant to inactivation. This leads to persistently low serum complement levels
and is associated with Type II membranoproliferative glomerulonephritis. C5 (choice B) is a
component of the complement system. C5a is an anaphylatoxin that effects vasodilatation in acute
inflammation. It is also chemotactic for neutrophils and monocytes and increases the expression of
adhesion molecules. A deficiency of C5a would affect the acute inflammatory response against any
microorganism or foreign substance. Secretory IgA (choice D) is the immunoglobulin associated with
mucous membranes. Selective IgA deficiency is the most common hereditary immunodeficiency. In
this disorder, there is failure of the B cell to switch the heavy chain class from IgM to IgA. Patients
have an increased incidence of sinopulmonary infections, diarrhea, allergies, and autoimmune
diseases.
- A sexually active 25-year-old man with epididymitis and orchitis demonstrates a prominent
leukocytic infiltrate with numerous neutrophils on biopsy. Which of the following organisms is
the most likely cause of this man’s infection?
A. Escherichia coli
B. Mycobacterium tuberculosis
C. Neisseria gonorrhoeae
D. Pseudomonas sp.
E. Treponema pallidum
The correct answer is C. Acute epididymitis and orchitis with prominent neutrophils in a sexually
active male are most likely caused by infection with Neisseria gonorrhoeae or Chlamydia trachomatis.
N. gonorrhoeae can produce a nonspecific pattern of acute inflammation (nonspecific epididymitis and
orchitis) or can be sufficiently severe as to cause frank abscesses within the epididymis.
Escherichia coli(choice A) is an important cause of nonspecific epididymitis and orchitis in children
with congenital genitourinary abnormalities and in older men. Mycobacterium tuberculosis(choice B)
can cause tuberculosis of the epididymis and testes, characterized by granuloma formation.
Pseudomonas sp.(choice D) has been implicated as an important cause of nonspecific epididymitis
and orchitis in older men. Treponema pallidum(choice E), the causative agent of syphilis, can cause
testicular involvement with (in later stages) gumma formation, endarteritis, or a prominent plasma cell
infiltrate.
- A 5-year-old child develops a febrile disease with cough, a blotchy rash, and cervical and
axillary lymphadenopathy. Also noted is an erythematous, maculopapular rash behind the ears
and along the hairline, involving the neck and, to a lesser extent, the trunk. Examination of this
patient’s oropharynx would likely reveal which of the following lesions?
A. Adherent thin, whitish patch on gingiva
B. Cold sores on the lips
C. Curdy white material overlying an erythematous base on the oral mucosa
D. Large shallow ulcers on the oral mucosa
E. Multiple small white spots on the buccal mucosa
The correct answer is E. The disease described is measles (rubeola), which has the typical
presentation described in the question stem. Measles is caused by a Morbillivirus, an RNA virus
belonging to the Paramyxovirus family. Koplik’s spots, which are pathognomonic for measles, are
small, bluish-white spots on the buccal mucosa in the early stages of measles. These lesions appear
just before the onset of the characteristic rash which is a brick red irregular maculopapular rash that
onsets 3-4 days after the onset of the prodrome (which can also involve the extremities) and fade as
the rash develops. This is usually a prodrome of fever, coryza, cough, conjunctivitis, malaise, irritibility
and photophobia as well as koplik’s spots. Leukoplakia is a premalignant condition characterized by
adherent whitish patches on the gingiva (choice A) and other sites in the oral cavity. Histologically,
they are similar to hyperkeratoses. Cold sores of the lips (choice B) are due to infection with herpes
viruses. Candida infection (thrush) produces curdy white material loosely attached to an erythematous
base (choice C). When removed there is typically a painful lesion that may bleed. Aphthous ulcers
are large shallow ulcers of the oral mucosa (choice D), commonly known as canker sores. No
systemic involvement is seen.
- A 38-year-old woman vacationing in Connecticut is bitten by a tick and develops chronic
arthritis of the knee and hip joints and paralysis of the left facial muscles. A physical
examination during the early stages of the disorder would most likely have revealed
A. aphthous ulcers in the mouth
B. erythema chronicum migrans
C. flaccid paralysis of limb flexors
D. purpuric lesions in a bathing trunk distribution
E. spastic paralysis of limb extensors
The correct answer is B. Lyme disease should be suspected in a patient who is bitten by a tick in the
northeastern United States. Lyme disease was named after a township in eastern Connecticut where
the disease was endemic. The disease is spread by way of a tick vector that transmits a spirochete
that causes a systemic illness. Erythema chronicum migrans is usually the first sign of the illness. This
is a large red patch on the buttocks or chest that slowly expands as the center blanches. Generally,
patients also have constitutional symptoms, such as fever and chills, during this phase. Stiff neck may
develop, along with other signs of meningeal irritation, because of an aseptic meningitis. Other
neurologic complications of Lyme disease include Bell palsy caused by involvement of branches of
the facial nerve. Arthritis is a prominent feature in approximately half the patients with Lyme disease.
It tends to appear several months after the infection but may persist for several years. The course of
the chronic arthritis shows exacerbations and remissions; the most commonly affected joints are the
knees and hips. Cardiac abnormalities in Lyme disease include pericarditis and heart block.
Skin manifestations do not include aphthous ulcers (choice A). Flaccid or spastic paralysis of limbs
(choices C and E) does not accompany Lyme disease; neurologic involvement is generally limited to
cranial nerves and meningitis. Purpura (choice D) is associated with vasculitis and does not occur in
Lyme disease.
- A 31-year-old HIV-positive man develops a severe pneumonia. Lower respiratory tract
secretions stained with methenamine silver stain demonstrate cup-shaped cysts with sharply
outlined walls. Which of the following organisms is the most likely pathogen in this case?
A. Candida albicans
B. Giardia lamblia
C. Haemophilus influenzae
D. Pneumocystis carinii
E. Streptococcus pneumoniae
The correct answer is D. The organism described is Pneumocystis carinii, which is an opportunistic
parasite that seems to be more closely related to fungi than to protozoa. Its cyst form, when stained
with silver stains, has the distinctive appearance described in the question stem, and is typically found
in frothy material that occupies the lumen of alveoli. Pneumocystis pneumonia is a common infection
among AIDS patients, and is very uncommon in other clinical settings. Formerly, many AIDS patients
died with Pneumocystis pneumonia, but the combination of early drug treatment (with
trimethoprim/sulfamethoxazole or pentamidine) and prophylaxis (usually with
trimethoprim/sulfamethoxazole) has decreased the number of fatal infections. In severe cases,
Pneumocystis infection can sometimes be demonstrated in extrapulmonary sites. Candida
albicans(choice A) can infect the lung and stain with methenamine silver, but the respiratory tract
secretions would probably include fungal hyphae and yeast forms. Giardia lamblia (choice B) causes
diarrhea, rather than pneumonia. Haemophilus influenzae(choice C) and Streptococcus
pneumoniae(choice E) are bacteria that would not stain with silver stains and are not especially
common in HIV-positive patients.
14. A neonate is born in very poor condition, with a severe, generalized encephalitis. If fever is noted as well as temporal lobe involvement, which of the following viruses is the most likely pathogen in this setting? A. Eastern equine encephalitis virus B. Herpes simplex type II C. Herpes zoster-varicella virus D. Poliomyelitis virus E. St. Louis encephalitis virus
The correct answer is B. Viral causes of neonatal encephalitis include three members of the herpes
family of viruses: herpes simplex I, herpes simplex II, and cytomegalovirus. All three types can have
devastating effects on the neonate, with extensive CNS damage leading to mental retardation,
seizures, and focal neurologic problems. Acyclovir may be of some help in modifying these infections,
but both treatment and prognosis remain very problematic. Herpes simplex encephalitis presents with
nonspecific symptoms: a flu-like prodome followed by headache, fever, behavioral and speech
disturbances. A distinguishable feature is a propensity to involve the temporal lobe with mass effect
on imaging studies and temporal lobe seizure foci on EEG’s. Eastern equine encephalitis virus
(choice A) and St. Louis encephalitis virus (choice E) are causes of epidemic encephalitis but are not
the most likely cause of neonatal encephalitis. Herpes zoster-varicella virus (choice C), unlike herpes
simplex, is not usually a cause of neonatal encephalitis. Poliomyelitis virus (choice D) is a
gastrointestinally transmitted virus that is not usually encountered in neonates (or anyone else in the
U.S. currently).
- A 23-year-old woman with a history of sickle cell disease presents with fever and severe
bone pain localized to her left tibia. An x-ray film reveals a lytic lesion, and blood cultures
reveal infection. A bone culture grows gram-negative rods. Which of the following best
describes the infecting organism?
A. It is comma-shaped and sensitive to acidic pH
B. It is a facultative intracellular parasite
C. It is motile and does not ferment lactose
D. It is motile and oxidase positive
E. It is a nonmotile facultative anaerobe
The correct answer is C. The presence of sickle cell disease in a question stem is usually a
significant clue. This question tests the fact that patients with sickle cell anemia are more susceptible
to osteomyelitis caused by Salmonella. The patient’s fever, bone pain, and x-ray results indicate
osteomyelitis. Note, however, that Staphylococcus aureus (gram-positive coccus) is the most
common cause of osteomyelitis in sicklers and nonsicklers. If it has not been ruled out on bone
culture, you should have looked for it in the answer choices. Notice that you were required to know
more than just the organism’s name; you needed to know its distinguishing features. Choice C
describes Salmonella (a gram-negative rod) accurately. Salmonella exists in more than 1,800
serotypes and is known to contaminate poultry.
A comma-shaped organism that is sensitive to acidic pH (choice A) is Vibrio cholerae, a gramnegative
rod that causes severe enterotoxin-induced diarrhea, with “rice water” stools and
dehydration. The toxin acts by stimulating adenylyl cyclase to overproduce cAMP in the brush border
of the small intestine. A facultative intracellular parasite (choice B) is Legionella, a catalase-positive
gram-negative rod. It contaminates air-conditioning cooling towers and causes Legionnaire disease (a
type of pneumonia). A motile and oxidase-positive organism (choice D) is Pseudomonas, a gramnegative
rod with pili that sometimes produces a polysaccharide slime layer. P. aeruginosa is the
prototype and commonly colonizes the lungs of patients with cystic fibrosis. It is associated with bluegreen
pus. A nonmotile, facultative anaerobe (choice E) is Shigella, a gram-negative rod that does
not produce H2S. All Shigella contain an endotoxic lipopolysaccharide. The organism causes bacillary
dysentery, with abdominal cramps, fever, and mucoid, bloody diarrhea.
- A patient suffers an infarct in the territory of the middle cerebral artery. Pathologic
examination of the patient’s brain would likely show
A. caseous necrosis
B. coagulative necrosis
C. enzymatic fat necrosis
D. gangrenous necrosis
E. liquefactive necrosis
The correct answer is E. Liquefactive necrosis occurs in brain or other neural tissues and in
pancreatic tissue. In this type of necrosis, the tissue appears liquefied under the microscope, without
preservation of cell outlines. Liquefactive necrosis can also be seen in some bacterial infections,
especially those caused by pyogenic Staphylococci, Streptococci, or certain coliform bacteria.
Caseous necrosis (choice A) is generally an indication of infection by Mycobacterium tuberculosis.
6
The term caseous refers to the appearance of the tissue, i.e., soft, white necrotic areas that have a
cheese-like appearance. Microscopically, the necrotic areas are lightly eosinophilic (stain light pink),
with little or no discernible cellular detail. The eosinophilia reflects staining of residual cellular proteins.
Coagulative necrosis (choice B) is a more common type of necrosis, characteristic of anoxic injury
and most infarcts. Cellular outlines are preserved, but proteins are denatured, and the cells stain in an
eosinophilic manner. Enzymatic fat necrosis (choice C) is seen primarily with pancreatic injury when
pancreatic lipases are released and digest fat to form free fatty acids. These fatty acids complex with
calcium, resulting in the production of calcium soaps (saponification) in the pancreatic tissue or in
extrapancreatic fatty tissues (eg, omentum). In gangrene (choice D), bacterial infection is
superimposed on a background of massive necrosis and putrefaction.
- A 4-year-old girl presents with a maculopapular rash on her hands and feet and painful
ulcers distributed anteriorly on her lips, palate, tongue, and buccal mucosa. Systemic features
and lymphadenopathy are absent. Which of the following viruses is most likely to have caused
this disorder?
A. Coronavirus
B. Coxsackievirus type A16
C. Herpes simplex virus type 1
D. Parainfluenza type 3
E. Rhinovirus
The correct answer is B. Hand-foot-and-mouth disease is characterized by the appearance of ulcers
in the mouth and a maculopapular or vesicular rash on the hands and feet. It is most frequently
caused by coxsackievirus type A16, although other coxsackieviruses have occasionally been
implicated. The disease usually affects young children. Systemic features and lymphadenopathy are
absent, and recovery is uneventful.
Coronavirus (choice A) is a cause of the common cold. Herpes simplex virus type 1 (choice C)
causes a variety of diseases, including gingivostomatitis, pharyngotonsillitis, herpes labialis, genital
herpes, keratoconjunctivitis, and encephalitis. Parainfluenza virus (choice D) is responsible for croup.
Croup, or acute laryngotracheobronchitis, is an acute febrile illness with stridor, hoarseness, and
cough. Rhinovirus (choice E) is a member of Picornaviridae. It is the most common cause of the
common cold.
- Evaluation of an adult third world immigrant to this country demonstrates chronic
headaches, chronic mild nuchal rigidity, and chronic inflammatory infiltrate of the CSF with
lymphocytes, plasma cells, macrophages, and fibroblasts. If an acid-fast organism is
identified, which of the following is the most likely etiologic agent?
A. Herpes virus
B. Mumps virus
C. Mycobacterium tuberculosis
D. Neisseria meningitidis
E. Streptococcus pneumoniae
The correct answer is C. Although this may seem to be a difficult clinical question, by simply
knowing that mycobacteria are acid-fast organisms, one can answer the question. Nuchal rigidity
suggests meningitis. It is convenient to classify meningitis based on the cerebrospinal fluid (CSF)
findings: (1) acute pyogenic meningitis if neutrophils are markedly increased, (2) acute lymphocytic
meningitis if lymphocytes (alone) are markedly increased, and (3) chronic meningitis if lymphocytes,
plasma cells, macrophages, and fibroblasts are increased. This patient has chronic meningitis. The
classic cause of chronic meningitis is tuberculosis, whose etiologic agent is Mycobacterium
tuberculosis. Other causes include other indolent meningeal infections such as syphilis, brucellosis,
and chronic fungal infections. The granulomas that are characteristic findings in other tissues may or
may not be present in the meningeal tissue, and are usually not recognizable in CSF. Tubercular
meningitis is now uncommon in this country. In immigrants from third world countries, a history of
pulmonary tuberculosis may be distant, undiagnosed, or deliberately concealed. Herpes virus (choice
A) and mumps virus (choice B) are causes of acute lymphocytic meningitis. These organisms are not
acid-fast. Neisseria meningitidis(choice D) is a gram-negative bacteria. Streptococcus
pneumoniae(choice E) is a gram-positive bacteria. Both are causes of acute pyogenic meningitis.
Neither is acid-fast.
- A 45-year-old white man with a history of alcohol abuse and periodontal disease has a
spiking fever, chills, and lung consolidation. A chest x-ray shows a cavity in the right lower
lobe that has an air/fluid level. Based on the clinical presentation, which of the following would
be the most likely cause of the symptoms?
A. Anaerobic bacteria
B. Aspergillus fumigatus
C. Entamoeba histolytica
D. Staphylococcus aureus
E. Streptococcus pyogenes
The correct answer is A. Anaerobic bacteria derived from the oral flora in the clinical setting of
periodontal disease are the most common isolates from lung abscesses. Single lung abscesses are
the most common pattern, with the superior segment of a lower lobe or the posterior segment of an
upper lobe being affected most often. Prominent members of the oral anaerobic flora include
fusobacteria, prevotella, and aerobic spirochetes. Aspergillus fumigatus(choice B) presents in the
lung as hemorrhagic infarctions, aspergillomas (fungus balls) in cavitary tuberculosis cavities, or as
allergic bronchopulmonary aspergillosis. Entamoeba histolytica(choice C) is associated with
pulmonary abscesses as an extension of a liver abscess across the diaphragm. Staphylococcus
aureus(choice D) usually presents as multiple lung lesions in noncontiguous sites because the
spread is embolic. The source of the infection is usually tricuspid endocarditis in intravenous drug
abusers. Streptococcus pyogenes(choice E) typically produces a bronchopneumonia pattern
following an upper respiratory infection.
- A young mother takes her infant to the pediatrician, who notices the infant’s teeth are
yellow and brownish striped. The antibiotic this mother most likely took during pregnancy
A. inhibits aminoacyl-tRNA binding
B. inhibits peptidyl transferase
C. interferes with cell wall synthesis
D. is a large, cyclic lactone-ring structure
The correct answer is A. This question relates to an NDBE favorite side effect: the teeth mottling
that occurs when a child is exposed to tetracycline in utero. You should remember that tetracycline is
contraindicated in pregnancy and early childhood up to approximately age 8 years. Tetracycline is a
bacteriostatic drug that binds to the 30s subunit of ribosomes, preventing aminoacyl-tRNA from
binding with complementary mRNA. This inhibits peptide bond synthesis. Resistance is plasmidmediated.
Inhibition of peptidyl transferase (choice B) occurs with chloramphenicol, a broad-spectrum
bacteriostatic agent that binds to the 50s subunit of ribosomes. Resistance is plasmid-mediated. It has
high toxicity (gastrointestinal disturbances, aplastic anemia, and gray baby syndrome), so it is used
mainly in severe infections or as a topical agent. Interference with cell wall synthesis (choice C)
occurs with penicillins and cephalosporins, the beta-lactam antibiotics. Resistance to these drugs
appears in organisms that have developed beta-lactamases (penicillinases), enzymes that destroy the
beta-lactam ring of these medications. The wider spectrum ampicillin, amoxicillin, ticarcillin, and
carbenicillin are particularly penicillinase susceptible. Large, cyclic, lactone-ring structures (choice D)
describe the macrolides: erythromycin, azithromycin, and clarithromycin. They inhibit bacterial protein
synthesis by reacting with the 50s ribosomal subunit and preventing the release of the uncharged
tRNA. Resistance is plasmid-mediated. Common side effects include gastrointestinal irritation, skin
rashes, and eosinophilia. Erythromycin is a popular choice for patients with penicillin hypersensitivity.
It is a cytochrome p450 inhibitor and therefore must be used with caution in patients taking other
drugs.
- A 38-year-old woman complains of cold and painful fingertips, as well as difficulty
swallowing and indigestion. Physical examination shows a thickened, shiny epidermis over
the entire body, with restricted movement of the extremities, particularly the fingers, which
appear claw-like. Which of the following autoantibodies will likely be found in this patient’s
serum?
A. Anti-DNA topoisomerase I (anti-Scl-70)
B. Anti-double-stranded DNA (ds DNA)
C. Anti-IgG
D. Anti-Sm
E. Anti-SS-A
The correct answer is A. This patient has systemic sclerosis, also called scleroderma. Antibodies to
topoisomerase I (anti-Scl-70) occur in up to 70% of patients with diffuse systemic sclerosis, but only
rarely in other disorders. Systemic sclerosis is characterized intitially by excessive fibrosis and edema
of the skin, especially the hands and fingers, producing sclerodactyly (characteristic changes in the
fingers, which resemble claws). Raynaud phenomenon is common. The diffuse type of systemic
sclerosis generally spreads to include visceral organs, such as the esophagus (producing dysphagia),
the lungs (producing pulmonary fibrosis), the heart (leading to heart failure or arrhythmia), and the
kidneys (renal failure causes 50% of scleroderma deaths). A more restricted variant of systemic
sclerosis with a somewhat more benign course is CREST syndrome (Calcinosis, Raynaud syndrome,
Esophageal dysmotility, Sclerodactyly, and Telangiectasia), characterized by the presence of anticentromere
antibodies (although 10% of CREST patients have antitopoisomerase antibody also).
Anti-ds DNA (choice B) and anti-Sm (Smith antigen; choice D) are characteristic of systemic lupus
erythematosus (SLE) but are not common in patients with systemic sclerosis. Rheumatoid factor is an
autoantibody directed against IgG (choice C). It is found in patients with rheumatoid arthritis. Anti-SSA
(choice E) is typically seen in Sjogren syndrome (although it may also be seen in SLE).
- A young patient presents with bilateral swelling of the parotid gland, accompanied by
swelling of one testicle. Which of the following is the most likely diagnosis?
A. Gonorrhea
B. Mumps
C. Nonspecific epididymitis and orchitis
D. Syphilis
E. Tuberculosis
The correct answer is B. Mumps, once a common childhood disease, is now much less common
because of immunization. It still should be considered as a potential cause of parotitis in medically
underserved populations, however. It is caused by the paramyxovirus and is spread by droplet
infection. Although testicular involvement by mumps in school-aged children is extremely uncommon,
postpubertal males who develop this viral infection have a 20-30% chance of subsequently
developing acute interstitial orchitis, usually in one testis. Microscopically, the inflammatory reaction
consists of lymphocytes, plasma cells, and macrophages. Although the process can be locally
destructive, sterility does not usually develop because typically only one testicle is involved. The
incubation period is 14-21 days before the onset of symptoms. Patients initially present with painful,
swollen salivary glands, usually the parotid. In unvaccinated individuals, other tissues frequently
become involved. These tissues include the testes, pancreas, and meninges. Neglected gonorrhea
infection (choice A) of the epididymis and testis occurs in the setting of previous sexual activity and
would not usually cause parotitis. Nonspecific epididymitis and orchitis (choice C) occur in children
with genital urinary congenital abnormalities (gram-negative rods), in sexually active young adults
(Chlamydia and gonorrhea), and in older men (gram-negative rods and Pseudomonas). Syphilis
(choice D) can cause orchitis with either gumma formation or a leukocytic infiltrate with prominent
plasma cells. Tuberculosis (choice E) can cause granulomatous involvement with acid-fast
organisms of the epididymis and testis.
- Which of the following cell surface markers is required for lysis of IgG-coated target cells
(antibody-dependent, cell-mediated cytotoxicity, or ADCC) by natural killer cells?
A. CD3
B. CD16
C. CD19
D. CD21
E. CD56
The correct answer is B. CD16 is a cell surface marker used to identify natural killer (NK) cells
(lymphocytes lacking most T- and B-cell markers). CD16 is an Fc receptor for IgG, allowing the NK
cells to bind to the coated target cell during ADCC, facilitating lysis. CD3 (choice A) is a fivepolypeptide
cluster that represents the nonvariable part of the T-cell receptor complex. The variable
part is able to rearrange itself to adapt to specific antigens. NK cells are CD3-negative. CD19 (choice
C) is a B-cell marker. It is a signal-transducing molecule that is expressed in early B-cell
differentiation. NK cells are negative for CD19. C21 (choice D) is also a B-cell marker. It is a
complement receptor, and is also the same receptor the Epstein-Barr virus uses to bind to cells during
infection. CD56 (choice E) is an NK cell marker, but is not involved with ADCC.
- Which of the following organisms would most likely cause infection after a partial
sterilization procedure that killed vegetative cells but did not kill spores?
A. Chlamydia
B. Clostridium
C. Escherichia
D. Pseudomonas
E. Streptococcus
The correct answer is B. Although bacterial spores are extensively discussed in microbiology
courses, you should be aware that only Bacillus (aerobic) and Clostridium (anaerobic) species
produce spores. This means that the list of diseases related to bacterial spore formation is also small:
anthrax (Bacillus anthracis), some forms of food poisoning (Bacillus cereus), botulism (Clostridium
botulinum), tetanus (Clostridium tetani), gas gangrene (Clostridium perfringens and others), and
pseudomembranous colitis (Clostridium difficile). This is one of the reasons why some clostridium
infections are difficult to treat. With respect to dentistry, clostridium difficile induced colitis can be seen
following treatment with antibiotics, especially clindamycin. Remember, clindamycin is commonly
used to treat a variety of dental-related infections because it has excellent anaerobic coverage. Note
also that spores (bacillus stearothermophilus) are used to test autoclave function.
- An elderly man develops a vesicular rash localized to a narrow circumferential band on one
side of his chest. The rash is very painful and the vesicles are confluent with some ulceration.
No other significant findings are demonstrated on physical examination. Which of the
following diagnoses is most likely to be correct?
A. Chicken pox
B. Herpes simplex I infection
C. Herpes simplex II infection
D. Measles
E. Shingles
The correct answer is E. This is shingles, the recurrent form of herpes zoster infection, which is
usually (except in the case of immunosuppressed patients) confined to a single, unilateral dermatome.
Isolated vesicles may be seen outside the dermatome. The primary herpes zoster infection (chicken
pox) precedes the development of shingles by years or decades; the prevalence of shingles rises
steadily with age, to the point that 1% of people older than 80 years of age have the condition.
Shingles lesions are infections and should be considered an infectious hazard in the hospital setting.
Acyclovir can ameliorate the condition. Varicella, or chicken pox (choice A), is the primary form of
herpes zoster infection and affects face and trunk diffusely. It is generally seen in young patients. The
macules evolve to papules and vesicles then crust over; all stages are simultaneously present.
Herpes simplex I (choice B) affects oral and perioral sites. Herpes simplex II (choice C) primarily
affects genital sites, producing a painful set of beefy red lesions. Measles (choice D) causes a
blotchy rash, rather than a dermatomal one. Patients present with a maculopapular rash that is brick
red. It begins on the head and neck and spreads downward and outward. Koplik spots often appear
on the buccal mucosa.
- A 35-year-old man who recently traveled to a third world country develops chronic, severe
dysentery with ulceration of the cecum. Biopsy reveals 15-40 micron amoebae with ingested
erythrocytes and small nuclei with distinctive tiny central karyosomes. If hepatomegaly is a
late complication of this patient’s condition, which of the following organisms is the most
likely culprit?
A. Isospora belli
B. Cryptosporidium parvum
C. Entamoeba histolytica
D. Giardia lamblia
E. Microsporidia spp.
The correct answer is C.Entamoebahistolytica is the usual cause of intestinal amebiasis, and has
the microscopic features described in the question stem. A particularly helpful (but not always
present) feature of this organism is the presence of ingested red blood cells within the amoebae.
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These amoebae cause flask-shaped ulceration of the intestinal mucosa and submucosa, with a
particular propensity for involving the cecum and ascending colon. The disease manifestations range
from none (asymptomatic carriers) to mild chronic diarrhea, to severe, purging dysentery. In
symptomatic cases, the liver may develop destructive amoebic liver abscesses that tend to become
secondarily (and potentially life-threateningly) infected by bacteria. Patients initially present with
recurrent diarrhea and abdominal cramps. As the condition worsens severe colitis may be present.
Then hepatic involvement may occur in fulminant cases. Isospora belli (choice A), Cryptosporidium
parvum (choice B), and Microsporidia spp. (choice E) are commonly seen in AIDS patients. These
organisms can cause severe diarrhea. None of these organisms are amebae. Giardia lamblia(choice
D) is a small intestinal protozoa with a distinctive pear-shaped morphology that appears to have a
“face.”
27. Which of the following characteristics would help to differentiate Streptococcus agalactiae from Streptococcus pneumoniae? A. Alpha-hemolysis B. Carbohydrate capsule C. Cytochrome enzyme system D. Growth in bile E. Oxacillin sensitivity
The correct answer is A. Streptococci are usually initially speciated by their hemolytic capacity on
sheep blood agar. Beta-hemolytic streptococci include groups A, B, and D. S. agalactiae is the classic
group B streptococcus. The non-beta-hemolytic streptococci consist principally of the pneumococci
and the viridans group. These groups are both alpha-hemolytic. Streptococcus is a genus of grampositive
facultative cocci occurring in pairs or chains. The genus is separable into the pyrogenic
group, viridans group, enterococcus group, and lactic group. Because streptococci are so common, it
is essential that the dental student understand how to differentiate all the different species. Note that
alpha hemolysis produces a green halo around the colony in RBC culture, whereas beta hemolysis
produces a clear halo. Both S. agalactiae and pneumococcus have a carbohydrate capsule (choice
B), an important virulence factor and means of subtyping streptococcal species. None of the
streptococci use cytochrome enzymes (choice C). They derive all of their energy from the
fermentation of sugars to lactic acid. Neither pneumococcus nor S. agalactiae can grow in bile
(choice D). This ability is specific for the enterococcus group (group D) of streptococci. Both
pneumococci and S. agalactiae are usually treated with penicillin-type antibiotics, although group B
streptococci require a penicillinase-resistant type such as oxacillin (choice E).
- A 73-year-old woman with a history of diabetes presents with left ear pain and drainage of
pus from the ear canal. She has swelling and tenderness over the left mastoid bone. Which of
the following microorganisms is the most likely causative agent?
A. Haemophilus influenzae
B. Klebsiella pneumoniae
C. Mucor sp.
D. Pseudomonas aeruginosa
E. Streptococcus pyogenes
The correct answer is D.Pseudomonas aeruginosa causes malignant otitis externa, which is a
severe necrotizing infection of the external ear canal. Infection tends to spread to the mastoid bone,
temporal bone, sigmoid sinus, base of the skull, meninges, and brain. Patients at increased risk
include the elderly, those with diabetes, and the immunocompromised. Haemophilus
influenzae(choice A) produces a variety of clinical syndromes. H. influenzae is the third most
common cause of meningitis in children aged 1 month to 18 years. It is the most common cause of
acute epiglottitis, the most common cause of purulent bacterial conjunctivitis, and the second most
common cause of otitis media. Patients at risk include those with COPD and cystic fibrosis, alcoholics,
splenectomized patients, and young patients. Klebsiella pneumoniae(choice B) is a gram-negative
organism that produces a necrotizing pneumonia in diabetic patients and alcoholics. Patients typically
present with an abrupt onset of fever, shaking chills, and purulent, foul-smelling sputum. Mucor
(choice C) is a fungal infection that is particularly severe in the diabetic or immunocompromised
patient. In the acidotic diabetic, the fungus produces a life-threatening, invasive rhinocerebral
infection. The infection begins in the nasal passages, extends into the paranasal sinuses, and
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spreads through the cribiform plate to the frontal lobes of the brain. Patients typically complain of
headache, facial pain, and orbital swelling. Streptococcus pyogenes(choice E) causes bacterial
pharyngitis, otitis media, and sinusitis. It is also associated with toxin-related diseases and skin
infections.
29. A poor African community is experiencing an epidemic of severe hepatitis. The mortality among pregnant women is particularly high. Which of the following viruses is the most likely cause of the epidemic? A. Cytomegalovirus (CMV) B. Hepatitis A virus (HAV) C. Hepatitis C virus (HCV) D. Herpes simplex I E. Hepatitis E virus (HEV)
The correct answer is E. Although hepatitis E is not generally seen in this country, it is essential that
the student have an understanding of all forms of hepatitis. Hepatitis E is an important, and until
recently, unrecognized cause of epidemics of enterically transmitted (fecal-oral) acute hepatitis.
Hepatitis E is caused by a virus that occurs primarily in India, Asia, Africa, and Central America.
Infection is associated with a 10-20% mortality among pregnant women. This type of hepatitis needs
to be considered in patients traveling to and from endemic areas. CMV (choice A) can cause acute
hepatitis, but the disease is usually mild and often goes unrecognized, except in profoundly
immunosuppressed patients. Hepatitis A virus (choice B) is the major cause of epidemics of
enterically transmitted viral hepatitis, especially in the United States, but is not a significant cause of
mortality in pregnant women. Hepatitis C virus (choice C) is usually transmitted parenterally rather
than enterically and is not a significant cause of mortality in pregnant women. Herpes simplex (choice
D) usually causes significant hepatitis only in profoundly immunosuppressed patients.
- Several students ate lunch at a restaurant at which they all were served pork with
vegetables and fried rice. All of the students developed nausea, vomiting, abdominal pain, and
diarrhea within 4 hours of eating lunch. Which of the following is the most likely cause of
these symptoms?
A. Bacillus cereus
B. Clostridium botulinum
C. Clostridium perfringens
D. EHEC (enterohemorrhagic escherichia coli)
E. Staphylococcus aureus
The correct answer is A. Bacillus cereus produces a self-limited diarrhea caused by ingestion of the
preformed enterotoxin in contaminated fried rice (especially when reheated) and seafood. The
incubation period is typically approximately 4 hours. The degree of vomiting is usually greater than
that of diarrhea. B. cereus is also associated with keratitis, producing a corneal ring abscess.
Clostridium botulinum (choice B) produces a neurotoxin that blocks the release of acetylcholine,
resulting in a symmetric descending paralysis that may lead to respiratory complications causing
death. Symptoms include blurred vision, photophobia, dysphagia, nausea, vomiting, and dysphonia.
Most cases are associated with the ingestion of contaminated home-canned food.
Clostridium perfringens (choice C) produces a severe diarrhea with abdominal pain and cramping
(sometimes called “church picnic” diarrhea). The incubation period is 8-24 hours after ingesting
contaminated meat, meat products, or poultry. The meats have usually been cooked, allowed to cool,
and then warmed, which causes germination of the clostridial spores.
EHEC, enterohemorrhagic escherichia coli (choice D), produces a bloody, noninvasive diarrhea
caused by the ingestion of verotoxin found in undercooked hamburger at fast food restaurants. Some
patients develop a life-threatening complication called hemolytic-uremic syndrome. Staphylococcus
aureus (choice E) produces a self-limited food poisoning syndrome with nausea, vomiting, and
abdominal pain followed by diarrhea beginning 1-6 hours after ingestion of the enterotoxin. The
organism is found in foods such as potato salad, custard, milk shakes, and mayonnaise.
- Which of the following organisms is most likely to be implicated as a cause of urethritis
that persists after antibiotic therapy for gonorrhea?
A. Actinomyces
B. Chlamydia
C. Mycobacteria
D. Nocardia
E. Rickettsia
The correct answer is B.Chlamydia, unlike the other choices, is a type of sexually transmitted
disease, as is gonorrhea. Gonococcal infections typically produce profuse urethal discharge,
especially in men, yielding a positive smear. Fever, rash and arthritis can occur with disseminated
disease. Chlamydia, Mycoplasma, and Ureaplasma are not effectively treated by penicillins and
cephalosporins, and are important causes of post-gonococcal urethritis. Chlamydial urethritis can be
diagnosed by using fluorescent antibodies to identify inclusions in epithelial cells. Actinomyces(choice
A) is a mouth commensal that rarely causes a deeper oral infection. Mycobacteria(choice C) cause
chronic granulomatous diseases such as tuberculosis and leprosy. Nocardia(choice D) can cause
necrotizing pneumonia and disseminated disease. Rickettsia(choice E) cause typhus and Rocky
Mountain spotted fever.
32. Which of the following organisms would be most likely to cause an outbreak of enteritis in a day care center in the United States? A. Helicobacter jejuni B. Salmonella typhi C. Shigella species D. Vibrio cholerae E. Yersinia enterocolitica
The correct answer is E. Enteritis is an inflammation of the intestine, especially the small intestine.
Yersinia enterocolitica is an important cause of mini-epidemics of pediatric diarrhea. Adults can also
be affected, but less commonly than children. Some diarrheal cases are severe (and occasionally
fatal) and may be complicated by severe dysentery, appendicitis, or chronic relapsing ileocolitis that
may require antibiotics to shorten the course. Yersinia is a non-motile ovoid or rod shaped,
nonencapsulated gram-negative bacteria. Helicobacter jejuni (choice A) is an important cause of
hospital-acquired diarrhea, especially in immunocompromised individuals. Salmonella typhi (choice
B) causes typhoid fever. It is a gram-negative bacteria. Shigella species (choice C) causes epidemics
of dysentery in military camps and other close quarter areas. It is a gram-negative bacteria. Vibrio
cholerae (choice D) causes cholera. It is a gram-negative bacteria. Gastroenteritis can occur in the
United States upon eating improperly cooked seafood from other countries.
33. In a closed system, spores are formed during which of the following phases of bacterial growth? A. Decline phase B. Exponential phase C. Lag phase D. Log phase E. Stationary phase
The correct answer is E. Spore formation usually occurs during the stationary phase, when cell
growth ceases because of a developing lack of nutrients or accumulation of toxins. During the phase
of decline (choice A), the lack of nutrients and the accumulation of toxin become so severe that any
viable organisms usually die before they can form spores.
The exponential phase (choices B) and log phase (choice D) are descriptors for active growth
occurring after the lag phase and before the stationary phase. Many antibiotics are most effective in
this period.
The lag phase (choice C) is the initial period of adaptation, prior to growth, which occurs when
organisms are introduced to a new environment.
- On physical examination, a man has several disfiguring lesions on his face and loss of
cutaneous sensation to fine touch, pain, and temperature. An acid-fast organism is observed
in scrapings from a skin lesion. Which of the following organisms is the most likely cause of
this patient’s disease?
A. Bartonella henselae
B. Listeria monocytogenes
C. Mycobacterium avium-intracellulare
D. Mycobacterium leprae
E. Nocardia asteroides
The correct answer is D. The disease in question is leprosy, or Hansen disease. The key feature in
the description is the fact the organism is acid-fast. These acid-fast bacilli are commonly found in skin
lesions or nasal scrapings. Both of the mycobacteria, M. avium-intracellulare and M. leprae are
strongly acid-fast, that is, they retain the carbol fuchsin dye in the face of acid-alcohol decolorization.
M. leprae has a predilection for the skin and cutaneous nerves, thereby producing the symptoms of
depigmentation and anesthetic cutaneous lesions. This loss of peripheral nerve function leads to
many of the disfiguring features of the disease; because patients do not have normal pain sensation,
they sustain repeated injuries. In addition, the organism attacks cartilage and causes granuloma
formation in the skin, leading to some of the facial disfigurement. Patients often have a history of
residence in an endemic area in childhood.
Bartonella henselae(choice A) is a very small, gram-negative bacterium that is closely related to the
rickettsia, although it is able to be cultured on lifeless media. It is the cause of cat-scratch disease, a
local, chronic lymphadenitis most commonly seen in children. Listeria monocytogenes(choice B) is a
ubiquitous microbe that causes disease in more than 100 animal species. Although it is best known as
an agent of meningitis in the newborn, it is a cause of multiple other diseases. A characteristic feature
of these infections is the development of granulomas at the site of the infection. The organism is not
acid-fast. M. avium-intracellulare(choice C) causes tuberculosis-like pulmonary disease in the
immunosuppressed, primarily in HIV infected individuals. Nocardia asteroides(choice E) primarily
produces pulmonary infections in humans. The organism is considered to be “weakly” acid-fast.
- A 28-year-old HIV-positive man complains of pain on swallowing. Physical examination
shows white plaque-like material on his tongue and buccal mucosa, which is biopsied. The
man is diagnosed with acquired immunodeficiency syndrome (AIDS). With which of the
following agents is the man most likely infected?
A. Candida albicans
B. Cytomegalovirus
C. Herpes simplex I
D. Human herpesvirus 8
E. Human papilloma virus
The correct answer is A. The most common early form of oral lesions in HIV infected individuals
includes thrush, hairy leukoplakia, and aphthous ulcers. Candida albicans produces oral thrush, an
AIDS-defining lesion, which is common in acute HIV disease, and becomes increasingly common as
the CD4 + cell count falls, especially as the CD4 decreases to less than 300. As the CD4 count
decreases to less than 100, the esophagus, trachea, bronchi, or lungs may be infected. The lesions
are usually painless. Diagnosis is by demonstration of pseudohyphae using a wet smear with
confirmation by culture. Although cytomegalovirus (choice B) is associated with numerous clinical
scenarios in the AIDS population, including odynophagia (painful swallowing), it would not produce
white plaques on the oral mucosa. Herpes simplex I (choice C) produces vesicular lesions occurring
in clusters in the oral cavity. There is an increased risk for herpes infections in the AIDS group, but the
lesions do not resemble those described in the question. These lesions are commonly red and
extremely painful. Human herpesvirus 8 (choice D) is the causative agent of Kaposi sarcoma, a
malignancy arising from endothelial cells that appears as hemorrhagic nodules in different organ
systems. It is the most common cancer in the HIV infected population. Human papilloma virus (choice
E) is associated with a variety of lesions, including warts and intraepithelial neoplasias of the vulva
and cervix. It is associated with anal condyloma, which can occur in the AIDS population.
- A viral organism was isolated from a painful blister on the lip of a teenage girl. The agent
was found to double-stranded, linear enveloped DNA. The patient had a similar sore
approximately 2 months ago following a week long trip to the beach. Which of the following is
the most likely causative organism?
A. Adenovirus
B. Coxsackie virus
C. Herpes simplex type 1 virus
D. Herpes zoster virus
E. Papilloma virus
The correct answer is C. Herpes simplex often causes recurrent grouped vesicles on an
erythematous base. The Tzanck smear is positive for multi-nucleated epithelial giant cells. Herpes
simplex is an enveloped, linear DNA virus that is a very common infectious agent; most adults will
have anti-Herpes simplex antibodies in their serum, although many may never have had any clinical
signs of disease. The hallmark of this disease is these painful skin vesicles, often called “cold” sores,
or “fever” blisters to denote the precipitating event that preceded the appearance of the lesions. The
virus has a propensity to become latent in the host’s nervous tissue. Activation of the infection occurs
following mild trauma (e.g., a visit to the dentist), hormonal changes (e.g., menses),
immunosuppression (e.g., following organ transplantation), or may follow minor infections, stress or
sun exposure. Other, more serious, manifestations of disease include encephalitis, pneumonia, and
hepatitis; these are particularly likely to be seen in immunodeficient patients such as those with AIDS.
Adenoviruses (choice A) are naked, linear, double-stranded DNA viruses that cause acute, usually
self-limiting, influenza-like illnesses occurring in the fall and winter. The symptoms include pharyngitis,
fever, cough, and general malaise. Epidemic pharyngoconjunctivitis and pneumonia can occur in
closed populations such as military installations.
Coxsackie viruses (choice B) are naked, single-stranded, polycistronic viruses with an RNA genome.
They are divided into groups A and B based on their virulence. Coxsackie A causes herpangina and
hand-foot-and-mouth disease, while Coxsackie B is seen in patients with pleurodynia, myocarditis,
and pericarditis. Both groups cause upper respiratory infections, febrile rashes, and meningitis.
Herpes zoster (choice D), the varicella virus, is an enveloped, double-stranded DNA virus that is a
very common infectious agent in children. Chickenpox is a mild, self-limiting illness in children that is
evidenced as a fever followed by a macular rash that progresses to papules, then vesicles of the skin
and mucous membranes. Shingles is a recurrence of a latent varicella infection in which the virus has
taken refuge in sensory ganglia of spinal or cranial nerves. Various factors that decrease the immune
status of the patient contribute to the exacerbation of the infection. Severe dermatomal pain occurs
with a vesicular eruption, fever, and malaise.
Papilloma viruses (choice E) are members of the Papovavirus family. They are non-enveloped and
possess a double-stranded, circular DNA genome. They cause skin, plantar, and genital warts.
- Electron micrographs of glomeruli reveal prominent deposits between the podocytes and
the basement membrane of the glomerular capillaries. If the patient was noted to have a skin
infection before this condition, these findings are most likely related to prior infection with
which of the following genera?
A. Escherichia
B. Klebsiella
C. Neisseria
D. Pseudomonas
E. Streptococcus
The correct answer is E. The disease is postinfectious glomerulonephritis, which is usually related to
prior sore throat or skin infection by Streptococcal species; Staphylococcus also causes some cases.
The prominent deposits are also known as subepithelial humps, and consist of immunoglobulin and
complement. Postinfectious glomerulonephritis commonly occurs after impetigo. Onset occurs within
1-3 weeks after infection (average 7-10 days). Other causes of postinfectious glomerulonephritis
include bacteremic states, such as systemic staphylococcus infections.
15
Gram-negative rods such as Escherichia(choice A), Klebsiella (choice B), and Pseudomonas
(choice D) have not been implicated as significant causes of postinfectious glomerulonephritis.
The gram-negative cocci Neisseria(choice C) have not been implicated in postinfectious
glomerulonephritis.
- Which of the following interleukins is produced by macrophages and stimulates fever
production by its action on hypothalamic cells?
A. IL-1
B. IL-2
C. IL-3
D. IL-4
E. IL-5
The correct answer is A. IL-1 is produced by macrophages and other antigen-presenting cells. It has
several actions, including stimulation of T cells to secrete IL-2, chemotactic activity for neutrophils and
monocytes, increased expression of intercellular adhesion molecules (ICAMs) on vascular endothelial
cells, and activation of macrophages and natural killer (NK) cells. It is also pyrogenic (fever inducing)
because of a direct effect on the hypothalamus.
IL-2 (choice B) is produced by activated T helper cells. It stimulates the proliferation of other T cells
as well as activated B cells. It also activates NK cells and stimulates lymphokine secretion.
IL-3 (choice C) stimulates all stem cells to produce hematopoietic cells; it is also known as
multilineage colony stimulating factor (CSF).
IL-4 (choice D) is produced by T helper 2 (Th 2) cells and mast cells. It has several functions,
including inducing cells to express class II major histocompatibility complex (MHC) antigens and
stimulation of B-cell proliferation. It is involved in the induction of atopic allergies by its mitogenic
activity for mast cells and its enhancement of immunoglobulin class switching to IgG and IeE.
IL-5 (choice E) is secreted by activated T helper cells. It promotes B-cell proliferation, production of
eosinophils, and stimulates B-cell class switching to IgA.
- A patient received second- and third-degree burns over his body. He later developed a
wound infection with a bluish-green exudate. Treatment with chloramphenicol and tetracycline
was unsuccessful. A gram-negative, motile organism was isolated. Which of the following
organisms was most likely isolated?
A. Candida albicans
B. Clostridium perfringens
C. Escherichia coli
D. Pseudomonas aeruginosa
The correct answer is D. Although this is a clinically-based question, it is essentially asking about
the microbiology of each bacteria. Pseudomonas aeruginosa is a very common opportunist in burn
patients and patients on ventilators, in whom it classically causes secondary wound infections and
septicemia. It may also cause cystitis in patients with urinary catheters and pneumonia in patients with
cystic fibrosis. The organism is found in water and usually gains access to the body as a contaminant
in the water used in respirators or in water baths, etc., used to cleanse wounds. This organism is a
nonfermenter, that is, it does not metabolize sugars by classic pathways. It produces a blue-green,
water-soluble pigment (pyocyanin), and has a fruity odor when grown on laboratory media. It has a
propensity for developing antibiotic resistance; current therapy uses the synergistic combination of an
aminoglycoside, such as amikacin, with a cell wall synthesis inhibitor (carbenicillin, ticarcillin, or
piperacillin).
Candida albicans (choice A) is a normal flora yeast that appears as large, weakly gram-positive,
spherical to ovoid organisms with budding daughter cells in Gram-stained preparations. Candidiasis is
an opportunistic infection in individuals with a compromised immune system. The fungus usually
causes mucocutaneous lesions, but in severely compromised individuals like patients with AIDS,
systemic disease may occur. Oral candidiasis appears as creamy white patches of exudate that can
be scraped off an inflamed tongue or buccal mucosa to reveal a painful lesion that may bleed.
16
Clostridium perfringens(choice B) is a gram-positive, spore-forming, anaerobic rod. It is a common
cause of gas gangrene when introduced into a wound. The organism produces a variety of toxins and
enzymes that enable it to destroy muscle tissue and spread through the soft tissues of the body.
Escherichia coli(choice C) is a lactose-fermenting, gram-negative rod seen as normal flora of the
intestine. It is the most common cause of urinary bladder infections, pyelonephritis, and sepsis in
patients with indwelling urinary catheters. It is also the major cause of traveler’s diarrhea and is a very
important pathogen in neonates, who become infected during passage through the birth canal.
- An 8-year-old girl is bitten in the leg by a neighbor’s cat. She presents the next day with
fever and bone pain localized to her right calf. An x-ray film reveals a lytic lesion of the right
tibia. Which of the following is the most likely pathogen?
A. Brucella melitensis
B. Eikenella corrodens
C. Francisella tularensis
D. Pasteurella multocida
E. Yersinia pestis
The correct answer is D. This patient has osteomyelitis due to a cat bite that penetrated the
periosteum. Whenever you see dog or cat bites in a question stem, consider Pasteurella multocida as
a primary cause of wound infection. This organism is a short, encapsulated, gram-negative rod
demonstrating bipolar staining. Rapidly arising cellulitis is particularly indicative of this organism.
Brucella melitensis (choice A) is a small, acapsular, gram-negative rod that causes brucellosis
(undulant fever) and is associated with contact with goats or sheep. Brucella abortus and Brucella
suis are variants associated with contact with cows and pigs, respectively. The organisms enter the
body through the skin or through contaminated dairy products, such as unpasteurized imported goat’s
milk or cheeses.
Eikenella corrodens(choice B) is commonly found in human bites. It is a gram-negative rod that is
part of the normal flora of the human mouth. Francisella tularensis(choice C) is a small, pleomorphic,
gram-negative rod that causes tularemia (rabbit fever). It occurs most commonly in rural areas. In the
U.S., rabbits are the main reservior for this organism, which is transmitted to humans by the
Dermacentor tick or by contact with infectious animal tissues. Yersinia pestis(choice E) is responsible
for bubonic plague, which has been known to occur in the western U.S. Its main reservoir is the prairie
dog, and its vector is the rat flea.
- A 27-year-old IV drug user presents with difficulty swallowing. Examination of the
oropharynx reveals white plaques along the tongue and the oral mucosa. Which of the
following best describes the microscopic appearance of the microorganism responsible for
this patient’s illness?
A. Budding yeast and pseudohyphae
B. Encapsulated yeast
C. Mold with nonseptate hyphae
D. Mold with septate hyphae
The correct answer is choice A. The patient has Candida esophagitis. Any time a patient presents
with dysphagia or odontophagia, along with whilte plaques in the oropharynx (thrush), you can
assume that the Candida is affecting the esophagus as well. The fact that the patient is an IV drug
user make an opportunistic infection such as Candida more likely. Candida appears as budding yeast
with pseudohyphae in vivo. The other answer choices represent the morphology of other important
opportunistic fungi: Cryptococcus are encapsulated yeast (choice B). You should think about
Cryptococcus neoformans when you’re presented with an immunocompromised patient with
neurologic symptoms. The classic clue is the presence of encapsulated organisms observable in an
India ink preparation. Mucor and Rhizopus are molds with nonseptate hyphae (choice C). You should
think about Mucor when you are presented with a diabetic (especially ketoacidotic) or a leukemic
patient with a severe sinus infection. Aspergillus is a mold with septate hyphae (choice D). In
immunocompromised patients, aspergillosis can present with acute pneumonia, often with a cavitation
(aspergillomas = fungus balls in the lungs).
- A 32-year-old woman with increased frequency of urination, suprapubic pain, and dysuria
for the past 3 days comes to the emergency department. She has no fever, nausea, or
vomiting. A Gram stain reveals gram-negative rods. Which of the following is the most likely
pathogen?
A. Escherichia coli
B. Neisseria gonorrhoeae
C. Shigella dysenteriae
D. Streptococcus pneumoniae
E. Treponema pallidum
The correct answer is A. This patient has the symptoms of a urinary tract infection (UTI).
Escherichia coli is the leading cause of community-acquired UTIs. The proximity of the urinary tract to
the anus facilitates colonization of the tract by fecal flora. Other gram-negative rods causing UTIs
include Enterobacter cloacae, Klebsiella pneumoniae, Serratia marcescens, Proteus mirabilis, and
Pseudomonas aeruginosa. None of the other choices listed cause UTIs.
- A 24-year-old AIDS patient develops chronic abdominal pain, low-grade fever, diarrhea, and
malabsorption. Oocysts are demonstrated in the stool. Which of the following organisms is
most likely to be the cause of the patient’s diarrhea?
A. Chlamydia psittaci
B. Entamoeba histolytica
C. Giardia lamblia
D. Isospora belli
E. Microsporidia
The correct answer is D. All of the organisms listed are protozoa. There are two intestinal protozoa
specifically associated with AIDS that can cause transient diarrhea in immunocompetent individuals
but can cause debilitating and potentially life-threatening chronic diarrhea in AIDS patients. These
organisms are Isospora belli, treated with trimethoprim-sulfamethoxazole (or other folate antagonists)
and Cryptosporidium parvum (no treatment presently available).
Chlamydia psittaci (choice A) is associated with the development of fever, chills, cough, and atypical
pneumonia with slightly delayed appearance of signs of pneumonitis. Entamoeba histolytica (choice
B) and Giardia lamblia(choice C) are both causes of diarrhea, but they are not specifically associated
with AIDS. Giardiasis, for example, can cause mild to severe bulky, greasy, frothy, malodorous stools,
free of blood and pus. Microsporidia (choice E) are a protozoan cause of diarrhea, but produce
spores rather than oocysts.
- A middle-aged man presents with fever, anorexia, and jaundice. He is diagnosed with
hepatitis caused by an enveloped DNA virus. What is the most likely causative organism?
A. Hepatitis A
B. Hepatitis B
C. Hepatitis C
D. Hepatitis E
The correct answer is B. Hepatitis B is a hepadnavirus, which is enveloped DNA. Hepatitis A
(choice A) is a picornavirus and is a naked-capsid RNA. Hepatitis C (choice C) is a flavivirus, which
is RNA enveloped. Hepatitis E (choice D) is a Calicivirus, which is a naked-capsid RNA.
45. A 6 month-old child who has been breast-fed since birth develops voluminous, non-bloody, watery diarrhea and vomiting. Which of the following viruses is the most likely cause of the child's diarrhea? A. Coronavirus B. Lymphocytic choriomeningitis virus C. Norwalk agent D. Orbivirus E. Rotavirus
The correct answer is E. Rotavirus is the major cause of diarrhea in infants and children under the
age of 2. It is a major cause of diarrheal morbidity worldwide due to dehydration. The virus replicates
in the intestinal mucosa, producing a profuse, watery, non-bloody diarrhea, often coupled with nausea
and vomiting. Transmission is by the fecal-oral route. Treatment is symptomatic with fluid and
electrolyte replacement. Coronaviruses (choice A) usually cause cold-like illnesses. Lymphocytic
choriomeningitis virus (choice B) can cause headache, malaise, myalgia, conjunctivitis, and,
occasionally, meningitis. Norwalk agent (choice C) can also cause diarrhea, but usually affects
patients older than 2 years. This causes profuse vomiting and diarrhea and is often transmitted by
food, especially shellfish. Orbivirus (choice D) is the cause of Colorado tick fever, which is the only
tick-borne viral disease in the United States.
- With respect to the bacterial spectrum of coverage, which of the following antibiotics has
the broadest activity against gram-negative organisms?
A. Cefuroxime (second generation cephalosporin)
B. Cephalexin (first generation cephalosporin)
C. Ceftriaxone (third generation cephalosporin)
D. Metronidazole
E. Vancomycin
The correct answer is C. For NBDE Part 1, it is important to know general properties of antibiotic
families. Ceftriaxone is a third generation cephalosporin commonly used in the treatment of a variety
of life-threatening infections caused by gram-negative organisms. The agent has moderate grampositive
activity. A general rule to remember about the cephalosporins is that when progressing from
first generation to third generation, the gram-negative activity is increased and gram-positive activity is
decreased. In other words, first generation cephalosporins have the most activity against grampositive
organisms and third generation cephalosporins have the most activity against gram-negative
organisms. Metronidazole (choice D) is an antibiotic with excellent anaerobic bacterial coverage.
Vancomycin (choice E) is an antibiotic with excellent gram-positive coverage; it is generally used in
the treatment of life-threatening infections.
47. If a newborn child develops an infection with Chlamydia trachomatis, how does infection with this organism cause blindness? A. Cataract formation B. Hemorrhage into the anterior chamber C. Hemorrhage into the posterior chamber D. Retinal detachment E. Scarring of the cornea
The correct answer is E.Chlamydiae are obligate intracellular parasites. Studies in the United States
demonstrate that 5-25% of pregnant women have C. trachomatis infections of the cervix. In these
women, approximately one half to two thirds of infants are exposed and if untreated can develop
inclusion conjunctivitis. The lesions begin with formation of lymphoid follicles in the conjunctiva. With
disease progression, there is tissue necrosis, granulation tissue deposition, and scar formation,
leading to lacrimal duct obstruction and distortion of the eyelids. With the loss of an adequate tear
system, the cornea becomes vulnerable to dehydration and opacification. Also, the vigorous
inflammatory response can directly involve the cornea, with resulting opacity.
49. Which of the following is a feature of gram-positive bacteria rather than gram-negative bacteria? A. Lipid A-containing lipopolysaccharide B. Lipoprotein in periplasmic space C. Outer membrane D. Peptidoglycan in periplasmic space E. Thick peptidoglycan cell wall
The correct answer is E. Most of the features listed are those of gram-negative bacteria, which have
a complex cell envelope consisting of a cytoplasmic (inner) membrane, a periplasmic space
containing peptidoglycan (choice D) and lipoprotein (choice B), an outer membrane (choice C), and
sometimes a capsule. The outer membrane contains lipopolysaccharide (choice A) which is a major
component of endotoxin. The peptidoglycan cell wall of the gram-negative bacteria is thin, while that
of the gram-positive bacteria is thick. Other features of gram- positive bacteria include a fairly simple
surface of cytoplasmic membrane, peptidoglycan, cell wall, and sometimes, an outer capsule. The cell
wall contains lipoteichoic acids. Gram-positive bacteria retain the stain or resist decolorizationby
alcohol in Gram’s method of staining. This is primarily characteristic of bacteria whose cell wall is
composed of peptidoglycan or teichoic acid.
- A diabetic patient has chronic sinusitis, which is eventually treated by evacuation of the
contents of the maxillary and ethmoid sinuses. Mucor species are found when the material is
examined histologically. The pathologist should notify the clinician immediately because
Mucor can cause a virulent
A. meningitis
B. pneumonia
C. septicemia
D. skin infection
E. urinary tract infection
The correct answer is choice A. Mucormycosis, to which diabetics are particularly vulnerable
(perhaps because of the high glucose content of the diabetic nasal secretions), typically involves the
nasal sinuses. It is especially dangerous because the organism can erode into the bones of the
cranium, causing life-threatening meningitis and/or encephalitis that is very difficult to treat. Mucor can
cause pneumonia (choice B), but this is not the most immediate threat. Mucor can cause septicemia
(choice C), but this usually occurs in more seriously ill patients and is not an immediate risk in this
patient. Mucor can cause a skin infection (choice D), but this generally occurs in conjuncition with
severe burns. Mucor does not usually cause urinary tract infections (choice E).
51. In trying to sterilize some dental materials, the dentist notices that one of the materials melts at 115 C. Which of the following sterilization methods is most appropriate for this material? A. Steam Autoclave B. Dry heat autoclave C. Glutaraldehyde (Cidex) soak (6 hours) D. Ethylene oxide E. Ethyl alcohol
The correct answer is choice D. Note firstly that ethyl alcohol and glutaraldehyde are chemicals that
can only high level disinfect, and cannot be used for sterilization. In particular, they are not sporicidal.
Choices A, B and D are the three most common methods of instrument sterilization. However, the
steam autoclave operates at 121 C, and the dry heat sterilizer at 160 C, both too hot for the material
which melts at 115 C. So we can only use a non-heat sterilization method. The only non-heat method
is ethylene oxide, at 8-12 hours of contact time.
- Global eradication of Lyme disease is unlikely because
A. Borrelia burgdorferi can be maintained in nature indefinitely by a tick vector
B. Borrelia burgdorferi is resistant to antibiotics and disinfectants
C. Borrelia burgdorferi is resistant to environmental stresses
D. human disease may reactivate after the primary infection (Brill-Zinsser disease)
E. humans are the primary reservoir for Borrelia burgdorferi
The correct answer is A. Borrelia burgdorferi can be maintained in nature indefinitely by a tick
vector. This organism is the tick-transmitted spirochete that causes Lyme disease. The tick, Ixodes
dammini, can infect the white-footed mouse and large mammals such as deer during its life cycle,
making them reservoirs. The tick itself is a reservoir, however, because it acquires the disease
through transovarial passage of the organism. Together, these factors make Lyme disease an
endemic infection with little hope for eradication. Lyme disease is characterized by erythema migrus,
a flat or slightly raised red lesion that expands with central clearing. Headache or stiff neck are
common. Arthritis is common, chronic, and recurrent. Borrelia burgdorferi is not resistant to antibiotics
and disinfectants (compare with choice B). The spirochete can be successfully treated with
penicillins, tetracycline, and ceftriaxone. Borrelia burgdorferi is a delicate spirochete and is not
resistant to environmental stresses (compare with choice C). Brill-Zinsser disease (choice D) is the
reactivation of epidemic typhus infection caused by Rickettsia prowazekii. It can occur many years
after an infection that was not treated with antibiotics. Humans are incidental hosts, rather than the
primary reservoir, for Borrelia burgdorferi(choice E). The primary reservoirs are ticks, mice, and large
mammals.
- A chef cut his finger. After a week, the site of the injury is warm, red, and swollen, and
begins draining pus. He contaminates some pastries with drainage from the lesion which later
were eaten by patrons of the restaurant. Within 4 hours they developed diarrhea and vomiting
with no fever. Which of the following organisms would be most likely to cause these
symptoms?
A. Bacillus cereus
B. Clostridium perfringens
C. Escherichia coli
D. Shigella sonnei
E. Staphylococcus aureus
The correct answer is E. Cellulitis is an acute infection involving the epidermis and dermis. Cellulitis
can lead to the development of an abscess, as seen in this patient. The most common types of skin
infections (cellulitis or abscess) usually involve gram-positive organisms such as staphylococcus and
streptococci. The chef had a staphylococcal abscess on his finger. S. aureus, produces enterotoxin A,
which was likely transferred to the pastries. When ingested, the toxin causes severe nausea and
vomiting within a few hours (the average incubation time is 3-6 hours). There is little diarrhea
associated with this type of food poisoning outbreak.
Bacillus cereus(choice A) is a gram-positive spore-forming rod that is associated with food poisoning
outbreaks following the ingestion of fried rice. The time of onset and symptoms would mimic
staphylococcal disease; the major differentiating feature is the food involved. The organism survives
the boiling of the rice because it is a spore-former. It germinates as the rice cools, grows, and
elaborates an enterotoxin that is responsible for the nausea and vomiting characteristic of the
disease. Clostridium perfringens(choice B) is a gram-positive spore-forming anaerobe that can cause
a longer incubation (18-24 hour) food poisoning, typically with marked diarrhea. Once again, the
spores allow the organism to survive the heating process used in the preparation of the food. Both
Clostridium perfringens and Clostridium botulinum are associated with home-canned vegetable and
sausages.
21
The symptoms of Escherichia coli(choice C) food poisoning are usually watery diarrhea (traveler’s
diarrhea) with minimal nausea and vomiting, or a bloody diarrhea caused by enteroinvasive strains of
the agent. Also, E. coli would be an unlikely cause of the primary infection in the chef. Shigella sonnei
(choice D) causes enterocolitis characterized by fever, cramps, and diarrhea after an incubation
period of one to four days. Transmission is fecal-oral, associated with poor hygiene. A wide range of
foods has been implicated.
- A Native American man is brought to a rural hospital in New Mexico with severe
bronchopneumonia, chills, fever, and headache. One day later, the man complains of chest
pain and difficulty breathing, and coughs up blood-tinged sputum. Chest x-ray reveals patchy
infiltrates and segmental consolidation. Which of the following organisms is the most likely
cause of this man’s pneumonia?
A. Poliomyelitis virus
B. Clostridium perfringens
C. Paramyxovirus
D. Listeria monocytogenes
E. Yersinia pestis
The correct answer is E. Any previously healthy person in the southwestern United States who
develops septic shock or severe pulmonary disease should be evaluated for plague. Plague is not an
extinct disease, but is still encountered in sporadic cases in various places, including Asia, Africa,
parts of Europe, and the American Southwest. The causative organism is Yersinia pestis, which is
endemic in many wild animal populations, and can be transmitted to humans either by direct contact
or by arthropod bite. Human plague may take many forms, including pestis minor (mild
lymphadenopathy); bubonic plague (prominent lymphadenopathy); pneumonic plague (as described
above); and septicemic plague. Antibiotics are most effective if given within the first 24 hours, which
can be problematic if medical staff do not suspect the disease. Because plague is rare in the United
States, a high degree of clinical suspicion is required to make a rapid diagnosis and to institute timely
treatment. If the diagnosis is missed, the mortality rate is high.
Poliomyelitis virus (choice A) is an enterovirus that causes muscle weakness, headache, stiff neck,
sore throat, fever, nausea, and vomiting. Lower motor neuron lesions (flaccid paralysis) can be seen
as well as deep tendon reflexes and muscle wasting. Clostridium perfringens(choice B) causes gas
gangrene and gastroenteritis. This is the most common cause of gangrene. Paramyxovirus (choice
C) is the causative organism of the mumps. In this condition, exposure to paramyxovirus occurs 14-21
days before onset of symptoms. Patients present with painful swollen salivary glands, usually the
parotids. Frequent involvement of the other tissues is common, such as the testes, pancreas, and
meninges. Listeria monocytogenes(choice D) causes listeriosis. Infection during pregnancy may
result in sepsis, abortion, or premature delivery. Infection in the neonate may produce meningitis. In
immunocompromised adults, either meningitis or sepsis may occur.
- A patient with large, penetrating vegetations on his mitral and aortic valves develops
severe headaches. Acute bacterial endocarditis is diagnosed. Which of the following
organisms is the most likely cause of the patient’s disorder?
A. Herpesvirus
B. Mycobacterium tuberculosis
C. Staphylococcus aureus
D. Streptococcus pneumoniae
E. Treponema pallidum
The correct answer is C. The cardiac lesions described are characteristic of acute bacterial endocarditis. Intravenous drug users are among those susceptible to acute endocarditis. The fact the valvular vegetations are large and penetrating strongly suggests that they are caused by a virulent, pyogenic pathogen. Staphylococcus aureus is the most common cause of acute bacterial endocarditis. Brain abscess, which produces ring-enhancing radiologic lesions, is a known complication of bacterial endocarditis, occurring when the vegetations fragment and release septic emboli. The essential diagnostic characteristic for infective endocarditis includes a preexisting organic heart lesion, fever, new or changing heart murmur, evidence of systemic emboli, positive blood culture, and evidence of vegetation on endocardiography. 22 Herpesvirus (choice A) can cause encephalitis, but is not a cause of vegetative endocarditis. Mycobacterium tuberculosis(choice B) can cause brain masses, but does not usually cause endocarditis. Streptococcus pneumoniae(choice D) can cause acute pyogenic meningitis, but does not usually cause endocarditis. Treponema pallidum(choice E), the causative agent of syphilis, can cause aortic aneurysms and a variety of neurologic problems, but does not usually cause endocarditis or brain abscess.
- A can of disinfectant spray states that it kills HIV virus. This statement:
A. is important because of the difficulty in killing this virus
B. shows that the disinfectant will kill most other pathogens
C. illustrates the use of a “benchmark” organism
D. is not a good indication of disinfectant strength
The correct answer is D. HIV virus is notoriously easy to kill on most environmental surfaces. Many
disinfectants and other common physical and chemical exposures will kill the virus fairly easily.
Therefore, ability to kill HIV is NOT an important criterion in choosing disinfectants. Mycobacterium
tuberculosis is chosen as the “benchmark organism” (the standard against which the disinfectant is
compared). It is very resistant to surface disinfectants, partially because of its waxy cell wall. Another
highly resistant pathogen on environmental surfaces is Hepatitis A virus.
- A 35-year-old, sexually active man presents with a painless penile vesicle and inguinal
lymphadenopathy. The infecting organism is definitively diagnosed and is known to exist in
distinct extracellular and intracellular forms. Which of the following is the most likely
pathogen?
A. Calymmatobacterium granulomatis
B. Chlamydia trachomatis
C. Haemophilus ducreyi
D. Neisseria gonorrhoeae
E. Treponema pallidum
The correct answer is choice B. This patient has lymphogranuloma venereum caused by Chlamydia
trachomatis (type L1, 2, or 3). Chlamydia exhibit distinct infectious and reproductive forms. The
extracellular infectious form is known as the elementarly body (EB), which cannot reproduce. It
attaches to the host cell and enters through endocytosis. Once inside the cell, the EB is transformed
into the reticulate body (RB) within the endosome. The RB is capable of binary fission and divides
within the endosome; fusion with other endosomes occurs to form a single large inclusion. Eventually,
the RBs undergo DNA condensation and disulfide bond bridgings of the major outer membrane
protein, forming EBs. The EBs are then released. Note that C. trachomatis is responsible for several
sexually or perinatally transmitted diseases, including ocular trachoma (types A, B and C), neonatal
conjunctivitis, nongonococcal urethritis, cervicitis, and pelvic inflammatory disease (types D-K).
Calymmatobacterium granulomatis (choice A) is a gram-negative rod that causes superficially
ulcerated genital or inguinal papules that coalesce to form substantial lesions. The appearance of
Donovan bodies in histiocytes is diagnostic of this infection. Haemophilus ducreyi (choice C) is a
gram-negative rod that causes a soft, painful penile chancre, unlike that of a chlamydial or syphilitic
lesion. This infection is common in the tropics. Neisseria gonorrhoeae (choice D) is a gram-negative
diplococcus responsible for gonorrhea. Patients typically present with purulent penile discharge, not
genital lesion. Treponema pallidum (choice E) is the spirochete responsible for syphilis. It may cause
a firm, painless ulcer as a manifestation of primary syphilis, but the organism does not exist in distinct
extracellular and intracellular forms as does Chlamydia. Secondary syphilis is associated with the
appearance of condyloma lata – flat, gray wart-like lesions.
- A 23-year-old man develops explosive watery diarrhea with blood, fecal leukocytes, and
mucus approximately 3 days after eating chicken that was improperly cooked. Comma-shaped
organisms were also found in the fecal smear. Which of the following pathogens is the most
likely cause of these symptoms?
A. Campylobacter jejuni
B. Enterotoxigenic E. coli
C. Shigella sonnei
D. Staphylococcus aureus
E. Vibrio cholera
The correct answer is A. This is another question in which knowing the microbiological
characteristics is essential. Campylobacter jejuni is a pathogen causing an invasive enteric infection
associated with ingestion of raw or undercooked food products, or through direct contact with infected
animals. In the United States, ingestion of contaminated poultry that has not been sufficiently cooked
is the most common means of acquiring the infection. The patients typically have bloody diarrhea,
abdominal pain, and fever. The presence of fecal leukocytes indicates an invasive infection.
Campylobacter are microaerophilic, motile, gram-negative “comma-shaped” rods.E. coliare not
comma-shaped.
Enterotoxigenic E. coli(choice B) causes the classic traveler’s diarrhea. The infection is noninvasive
and is acquired by way of the fecal-oral route through consumption of unbottled water or uncooked
vegetables. The major manifestation is a copious outpouring of fluid from the gastrointestinal tract
presenting as explosive diarrhea. This is caused by the action of one of two types of enterotoxins on
the gastrointestinal tract mucosa. Shigella sonnei(choice C) produces a syndrome very similar to C.
jejuni. The organism, however, appears as a gram-negative rod on Gram’s stain. It does not have a
comma shape. Transmission is from person to person by way of the fecal-oral route. Infection
requires a low infective dose because the organism is fairly resistant to gastric acidity.
Staphylococcus aureus(choice D) produces food poisoning through the ingestion of a preformed
enterotoxin. The organism is present in food that is high in salt content, such as potato salad, custard,
milk shakes, and mayonnaise. The patient presents with nausea, vomiting, and abdominal pain,
followed by diarrhea beginning 1-6 hours after ingestion of the enterotoxin.S. aureus are seen as
clusters of coli and are not comma-shaped. Vibrio cholerae(choice E) produces a secretory diarrhea
caused by increases in cAMP in the intestinal cells. The organism is comma-shaped, but is not
invasive. The patient presents with the sudden onset of painless, watery diarrhea that becomes
voluminous, followed by vomiting. The stool appears nonbilious, gray, and slightly cloudy with flecks
of mucus, no blood, and a sweet odor.
- A 3-year-old boy presents with a 1-day history of loose stools, fever, abdominal cramping,
headache, and myalgia. He has no blood in the stool. A careful history reveals that he has had
several pet turtles. Which of the following is the most likely pathogen?
A. Chlamydia psittaci
B. Entamoeba histolytica
C. Salmonella spp.
D. Staphylococcus aureus
E. Yersinia enterocolitica
The correct answer is C. Salmonella spp., including S. enteriditis and S. typhimurium, produce a
gastroenteritis or enterocolitis. Patients with decreased gastric acidity, sickle cell disease, or defects in
immunity, and children younger than 4 years of age have a more severe course of disease.
Salmonella spp. are carried in nature by animal reservoirs such as poultry, turtles, cattle, pigs, and
sheep. The incubation period is 8-48 hours after ingestion of contaminated food or water. Chlamydia
psittaci (choice A) produces an interstitial pneumonitis accompanied by headache, backache, and a
dry, hacking cough. A pale, macular rash is also found on the trunk (Horder spots). Patients at risk
include pet shop workers, pigeon handlers, and poultry workers. Entamoeba histolytica (choice B)
produces a diarrhea (frequently bloody or heme-positive), right lower quadrant crampy abdominal
pain, and fever. Patients frequently have weight loss and anorexia. There is usually a history of travel
outside the United States. Most cases are chronic. Complications include liver abscesses.
Staphylococcus aureus (choice D) produces a self-limited gastroenteritis caused by the production of
preformed, heat-stable enterotoxins. The incubation period is 16 hours. The toxins enhance intestinal
peristalsis and induce vomiting by a direct effect on the CNS. Yersinia enterocolitica (choice E)
usually produces a chronic enteritis in children. These patients have diarrhea, failure to thrive,
hypoalbuminemia, and hypokalemia. Other findings include acute right lower quadrant abdominal
pain, tenderness, nausea, and vomiting. The infection mimics appendicitis or Crohn disease.
- A 48-year-old presents with malaise, loss of appetite, nausea, moderate fever, and
jaundice. Laboratory tests indicate a marked increase in serum transaminases and presence
of HBsAg, HBc IgM antibody, and HCV antibody. Antibody tests for HBsAb and HAV are
negative. The results indicate:
A. A dual infection of HBV and HAV
B. Chronic hepatitis A infection
C. Chronic hepatitis B infection
D. Hepatitis C infection
E. The presence of an acute HBV infection.
The correct answer is E.The presence of hepatitis B surface antigen (HBsAg) along with hepatitis B
core IgM antibody (HBc IgM Ab), and the absence of hepatitis B surface antibody (HBsAb) indicates
the presence of the early stages of an acute infection with Hepatitis B. The presence of antibody to
Hepatitis C (HCV) only indicates exposure, but not a specific time of exposure; however, 85% of
patients who are infected with HCV develop chronic infections, indicating that this patient has an 85%
chance of having a dual infection with HBV and HCV. A dual infection of HBV and HAV (choice A) is
not plausible since the IgM anti-HAV serology is negative. Hepatitis A does not cause chronic disease
(choice B). Chronic HBV infection (choice C) is unlikely because the patient has HBc IgM Ab, which
is characteristic of an acute infection, rather than a chronic infection. Hepatitis C infection (choice D)
is not confirmed by these data because the presence of HCV Ab only indicates exposure to the virus,
and not the state of infection. This could be caused by exposure at some earlier time (the elevated
serum transaminases might be due to HBV infection). An active or chronic HCV infection can only be
confirmed by PCR.
- A 47-year-old man presents with a nonproductive cough and rales. A chest x-ray film
suggests atypical pneumonia. The pneumonia resolves after treatment with azithromycin. A
diagnosis of psittacosis is established by the presence of complement fixing antibodies
against Chlamydia psittaci in the convalescent serum. Which of the following is the most likely
occupation of this patient?
A. Cat breeder
B. Florist
C. Homeless shelter worker
D. Poultry farmer
E. Slaughterhouse worker
The correct answer is D. When you see Chlamydia psittaci, one word should come to mind: birds!
Infection with this organism is an occupational hazard for anyone who works with birds (e.g.,
veterinarians, pet store employees), including poultry farmers. Individuals who work with cats (choice
A) would be at an increased risk for infection with Pasteurella multocida (which is acquired primarily
through cat bites), Bartonella henselae (cat-scratch fever), and the protozoa Toxoplasma gondii
(which can be acquired from ingestion of food contaminated with cat feces). Florists (choice B) are at
increased risk for infection with the fungus Sporthrix schenckii, a primary pathogenic fungus acquired
by inoculation (e.g., a rose thorn puncturing the skin). A person who works in a homeless shelter
(choice C) would be at increased risk for infection with Mycobacterium tuberculosis, because this
organism is spread through the air and is prevalent in the homeless community. Slaughterhouse
workers (choice E) are at an increased risk for infection with Brucella, a bacterium that is acquired by
handling infected animals.
