Micro USMLE 8-28(13) (qmax 8/23 - 6-40)

Question 1

This patient presents with fever, headache, malaise, and a non-productive cough. Given his occupation as a pet-store owner, ongoing exposure to birds should be considered as a risk factor for developing psittacosis, an atypical pneumonia caused by Chlamydophila psittaci. This bacterium has been documented in over 460 species of bird. Infection is usually acquired by inhalation of dried feces that becomes aerosolized when caged birds exercise their wings or in bird-feather dust. C. psittaci infections commonly present with the symptoms demonstrated by this patient. Chest x-ray typically reveals diffuse and bilateral patchy infiltrates. Histology of the specimen obtained by bronchoalveolar lavage would show the presence of ?

Answer 1

cytoplasmic inclusion bodies seen with Giemsa stain. Treatment of choice is a 10- to 14-day course of doxycycline.

Question 2

Intranuclear inclusions are typically seen in cytomegalovirus (CMV) and adenovirus. CMV typically occurs in immunocompromised hosts, whereas, adenovirus typically occurs in young children. Yersinia enterocolitica is seen on light microscopy as a gram-negative rod with bipolar staining and is transmitted via pet feces; however, it does not cause respiratory symptoms.
Pneumocystis jirovecii is a fungus with a fried-eggs appearance or disk shapes on methenamine silver stain. This pathogen causes bilateral and diffuse pneumonia, but typically only in immunocompromised patients.

Streptococcus pneumoniae is a gram-positive lancet-shaped diplococcus and is the leading cause of ?

Answer 2

community-acquired lobar pneumonia; in patients with Streptococcus pneumonia infection, lobar consolidation is seen on x-ray in contrast to the diffuse, bilateral, patchy infiltrates seen in this patient with an atypical pneumonia.

Question 3

Individuals exposed to birds are at risk of developing psittacosis caused by Chlamydophila psittaci. Histology in an infected patient shows ?

Answer 3

cytoplasmic inclusion bodies with Giemsa stain. The treatment of choice is a 10- to 14-day course of doxycycline.

Question 4

This boy is exhibiting symptoms of advanced rabies infection. His illness began shortly after a camping trip, where his cave-exploring could have exposed him to bats, raccoons, and other carriers of the rabies virus, an ?

Answer 4

enveloped single-stranded RNA virus. Exposure is typically followed by a prodromal illness in the days following. Prodromal symptoms may appear nonspecific, including low-grade fever, chills, myalgias, fatigue, anorexia, nausea, and headache

Rabies has a long incubation period, so the progression to severe disease and rabies encephalitis may take weeks to months, which is in line with this patient’s history. Hydrophobia is a very unusual and specific symptom of rabies, and it occurs due to dysphagia and severe involuntary pharyngeal muscle spasms when attempting to drink. This patient’s complaints of throat pain and aversion to oral intake are suggestive of such underlying pathology. Other late-onset symptoms include agitation, photophobia, and hypersalivation, which eventually lead to paralysis, coma, and death, if left untreated.

Question 5

The rabies virus is part of the rhabdovirus family, which are single-stranded, enveloped, and helical. The virus first binds to acetylcholine receptors before traveling in a retrograde fashion up the nerve axon. It makes its way to the CNS and targets the cerebellum and hippocampus.

Here is what you would expect if the patient was infected with the other pathogens listed:

Systemic infection by a dimorphic fungus commonly manifests with pulmonary symptoms, such as coughing, and a shorter incubation period.
An enveloped double-stranded DNA virus like herpes simplex virus (HSV) can product?

Answer 5

herpes encephalitis, but is not associated with the sore throat or weight loss seen in this scenario.
A gram-positive spore-forming bacterium like Clostridium tetani can cause tetanus, which might explain muscular spasms in the jaw but not the patient’s delirium or other symptoms.
Amebic encephalitis from the single-celled parasite Naegleria fowleri is quite rare and would have resulted in rapid deterioration within days.

Question 6

Rabies infection manifests early with a nonspecific prodrome, followed by a long incubation that slowly progresses to severe encephalitis with hydrophobia. The rabies virus is a member of the rhabdovirus viral family, which are ?

Answer 6

single-stranded, enveloped, and helical.

Question 7

The patient is bitten by a dog carrying Pasteurella multocida, a gram-negative coccobacillus (although there is some variation in morphology), which is part of the normal oral flora found in the mouths of cats and dogs. It causes an aggressive, rapidly spreading infection that can lead to skin abscesses, as seen in this patient. The distinct coccobacillus morphology of this infection, along with the clinical history, helps identify?

Answer 7

Pasteurella multocida as the correct answer.

Question 8

Bartonella henselae is a gram-negative rod that is usually associated with cat scratches and most often causes lymphadenitis in young children. Brucella canis, a gram-negative rod that is normally found in dogs, can cause fever, malaise, and hepatosplenomegaly in humans, but not usually purulence. Eikenella corrodens is a gram-negative, facultative, anaerobic bacillus found ?

Answer 8

in the human oral cavity. It is one of the HACEK organisms involved in endocarditis. Francisella tularensis is a gram-negative rod acquired via contact with infected rabbit tissue.

Question 9

The most common cause of cellulitis in a patient after a cat or dog bite is?

Answer 9

Pasteurella multocida, typically a gram-negative coccobacillus (with some variability in morphology).

Question 10

A patient with fulminant sepsis is brought to the emergency department and transferred quickly to the intensive care unit. Blood cultures yield two strains of virulent bacteria. Bacterial strain X is resistant to ampicillin and sensitive to gentamicin. Bacterial strain Y is resistant to gentamicin and sensitive to ampicillin. Bacterial strains X and Y are grown in mixed cultures, in medium without antibiotics, and then the culture is plated on medium containing both ampicillin and gentamicin. Bacterial colonies grow on the plates. In a second experiment, separate strains of X and Y are co-cultured in DNAse-containing antibiotic-free media, then plated on culture medium containing ampicillin and gentamicin. No colonies grow on these plates.

Assuming that bacterial cells are impermeable to DNAse, which of the following processes best explains these observations?

Answer 10

Transformation is gene transfer resulting from the uptake of DNA from the environment. Strain X has an ampicillin-resistance gene (AmpR), and strain Y has a gentamicin-resistance gene (GenR). During the initial coculture step of the first experiment (no DNAse), some bacteria die and release genetic material into the medium. The surviving bacteria tend to pick up this genetic material, as shown in the drawing. Therefore, some GenR bacteria picked up the AmpR gene, and vice versa; when this coculture was then plated on media containing both ampicillin and gentamicin, bacteria with both the AmpR and the GenR genes survived.

In the second experiment, when bacteria died and spilled their genetic material into the media, their DNA was destroyed by the DNAse, so bacteria did not have a chance to undergo transformation.

Question 11

Mutation (changes to the DNA that alter its coding) generates resistance at a lower rate for these antibiotics than seen in this experiment. .
Transcription is the first step of gene expression in which a particular segment of DNA is copied into RNA by an enzyme RNA polymerase.
Transduction is gene transfer from?

Answer 11

a donor to a recipient bacteria via a bacteriophage. It is not inhibited by DNAse in the medium.
Transposition elements are freely movable from one location to another. They are most commonly transferred by conjugation, which is not inhibited by DNAse in the medium, though transposable elements may also be moved by transformation.

Question 12

Transformation occurs when a cell takes up DNA from a donor cell. When two types of bacteria are grown in mixed culture, some might survive bactericides that could?

Answer 12

kill each type individually, unless DNAse is added to destroy DNA spilled into the medium before cells can absorb it.

Question 13

This patient has a 2-day history of fever, malaise, anorexia, conjunctivitis, coryza (catarrhal inflammation of the nasal mucosa), and cough. Koplik spots, the bright red spots with blue-white centers indicated on the image, are found on the patient’s buccal mucosa. He has not received any vaccinations. This presentation makes a diagnosis of a measles (rubeola) virus very likely. Koplik spots are a key finding because they are pathognomonic for measles. Following the appearance of Koplik spots, patients typically develop a maculopapular blanching rash that starts at the head and neck and travels downward 1–2 days later.

Measles virus is a member of the paramyxovirus family, which includes ?

Answer 13

enveloped, single-stranded, negative-sense, and linear RNA viruses. This family also encompasses parainfluenza virus, respiratory syncytial virus, and mumps virus.

Question 14

Corynebacterium diphtheria is a club-shaped gram-positive rod; initial symptoms of infection are sore throat, low-grade fever, malaise, and an adherent gray pseudomembrane over the tonsils, pharynx, and/or nasal cavity. Bordetella pertussis is a gram-negative coccobacillus and is characterized by an inspiratory whoop and posttussive vomiting.
Human herpesvirus 6 (HHV-6) is an ?

Answer 14

enveloped, double-stranded, and linear DNA virus like all herpes viruses and is seen mostly in children within the first 2 years of life. Rubella is an enveloped, single-stranded, positive-sense, and linear RNA virus; initial symptoms of infection are a maculopapular rash and minimal systemic symptoms.

Question 15

Measles (rubeola) virus is ?

Answer 15

an enveloped, single-stranded, negative-sense, and linear RNA virus. Measles virus manifests with Koplik spots and the 3 C’s: Cough, Coryza (runny/stuffy nose), and Conjunctivitis. The rash of measles virus typically progresses in a cranial-to-caudal fashion, beginning 1 to 2 days after the appearance of Koplik spots.

Question 16

This patient presents with a painful and swollen right knee, accompanied by a rash on the soles of his feet and a recent history of gastroenteritis. Combined with his laboratory test results, which show an elevated white blood cell (WBC) count, neutrophilic predominance, and a normal glucose level, the most likely diagnosis is reactive arthritis. (Normal characteristics of synovial fluid include a WBC count of <200 cells/µL, <25% polymorphonuclear neutrophils, high viscosity, and a glucose level similar to this patient’s serum glucose level.)
One of the seronegative spondyloarthropathies, reactive arthritis follows (is a reaction to) either a gastrointestinal (GI) or genitourinary (GU) infection and develops within 2 to 4 weeks of that infection. Reactive arthritis is characterized by asymmetric arthritis accompanied by other extra-articular manifestations, which may include the following:?

Answer 16

conjunctivitis, which often precedes the arthritis
keratoderma blennorrhagica, a papulosquamous skin rash on the palms and soles, like that shown in the image
urethritis, especially when the preceding illness is a GU infection
circinate balanitis, a lesion on the penile shaft
aortitis (occasionally), which becomes aortic valve regurgitation

Question 17

The classic presentation of reactive arthritis can be remembered with the mnemonic “can’t see (conjunctivitis), can’t pee (urethritis), can’t bend my knee (arthritis), and no anti-IgG.” Reactive arthritis is associated with human leukocyte antigen B27, as is psoriatic arthritis (an important finding from this patient’s family history). Because it is one of the seronegative spondyloarthropathies, no associated anti-IgG antibodies will be found.

Each of the other answer choices is associated with a different cause of arthritis. Anti-IgG antibodies may represent rheumatoid factor and would be associated with?

Answer 17

rheumatoid arthritis (RA). Heberden nodes are associated with osteoarthritis, which is a chronic process. Malar rash is classically associated with systemic lupus erythematosus (SLE). SLE may include both arthritis and a rash, but the location of the rash on the soles of the feet and the description of recent GI illness are more suggestive of reactive arthritis. Finally, ulnar deviation of the fingers is a late manifestation of RA, which is a less likely diagnosis, given this patient’s rash and recent GI illness.

Question 18

This infant has microcephaly, hearing loss, hepatosplenomegaly, and a petechial rash. Furthermore, the mother discloses that she had fever, sore throat, and fatigue early in her pregnancy. In this setting, the most likely diagnosis is ?

Answer 18

a congenital cytomegalovirus (CMV) infection. Pregnant women can become infected with CMV through sexual contact or organ transplantation. Pregnant women with CMV infection are typically free of symptoms but may have a mononucleosis-like illness, as described by this infant’s mother. Fetuses exposed to CMV during the first trimester may experience intrauterine growth retardation in addition to central nervous system damage with hearing and sight impairments. Intellectual disability can occur along with microcephaly. A classic feature of CMV is periventricular calcifications, which may be observed on a head CT scan.

Question 19

CMV is part of a group of microbes that cause ToRCHeS infections, alongside Toxoplasma gondii, Rubella, Cytomegalovirus, HIV, Herpes simplex virus-2, and Syphilis. Most ToRCHeS infections are transmitted from?

Answer 19

mother to fetus transplacentally; however, transmission during delivery (particularly with herpes simplex virus-2) is also possible in some cases. Any ToRCHeS infection can result in hepatosplenomegaly, jaundice, thrombocytopenia (petechial skin rash), and growth retardation.

Question 20

Congenitally acquired rubella is associated with cataracts, hearing loss, and congenital heart disease.
Congenitally acquired herpes simplex virus is associated with neonatal meningoencephalitis and vesicular, herpetic lesions.
Congenitally acquired HIV presents?

Answer 20

as recurrent infections and chronic diarrhea in neonates.
Congenitally acquired syphilis often results in stillbirth and hydrops fetalis. If the infant survives, he or she will have notched teeth, a saddle nose, saber shins, and sensorineural deafness.

Question 21

Congenital CMV infection manifests in neonates as ?

Answer 21

hearing loss, seizures, petechial rash, microcephaly, and evidence of periventricular calcifications on CT of the head. Maternal manifestation may include a mononucleosis-like illness during pregnancy.

Question 22

The patient is experiencing increased irritability, feeding difficulty, and other general nonspecific signs, along with a bulging fontanelle, characteristic of meningitis in neonates. The high opening pressure, high WBC count, and low glucose of the CSF is diagnostic of bacterial meningitis. The high pressure is the result of acute inflammation, resulting in the influx of white cells and fluid into the CSF (pleocytosis). Since bacteria are consuming glucose and less glucose is transported in the CSF, glucose values decrease, Meanwhile, the white cells also consume glucose and degranulate, resulting in the high protein value.
In infants 0–3 months old, the most common organisms causing meningitis are?

Answer 22

group B streptococci, Escherichia coli, and Listeria monocytogenes. Additionally, this infant is preterm and low birth weight, both of which increase the risk for L. monocytogenes meningitis because of increased exposure to hospital personnel.

Question 23

This organism displays two important characteristics relevant to its pathogenesis that separate it from other bacteria: (1) It can grow at a broad temperature range (1°C–45°C) in the presence of high salts, and (2) it has the capacity to invade and survive within eukaryotic cells. The intracellular, invasive capacity of L. monocytogenes at 37°C allows the organism to evade host immune mechanisms. They are not encapsulated; the hallmark of most bacterial (and fungal) pathogens causing meningitis express an immunologically cryptic, carbohydrate capsule. One other notable fact is that L. monocytogenes is highly motile at room temperature, allowing these organisms to expand their environment (swim to other infectious points).

Treatment for bacterial meningitis needs to be initiated without knowing the infectious source. Therefore, empiric antibiotic therapy for meningitis in the young infant should include?

Answer 23

the cell wall synthesis inhibitor (ampicillin) and an aminoglycoside such as gentamicin to cover for other common organisms.

Question 24

A nonenveloped, linear, single-stranded RNA virus with an icosahedral configuration refers to enterovirus and an enveloped, double-stranded, linear DNA virus refers to herpes simplex virus. Although both of these viruses can cause meningitis, the CSF profile would demonstrate a higher glucose and the WBC would mainly be lymphocytes, rather than neutrophils.
A polysaccharide capsule, oxidase positive, ferments glucose and maltose refers to?

Answer 24

Neisseria meningitidis, which also causes bacterial meningitis but is commonly seen in older children.
Bile-soluble, optochin sensitive, with a positive quellung test refers to Streptococcus pneumoniae, which is a common cause of bacterial meningitis in older age groups. However, it is seen less frequently in young infants ages 0–3 months.

Question 25

Neonatal bacterial meningitis is characterized by an elevated intracranial pressure, a high white cell count, a high protein concentration, and a low glucose concentration. Listeria monocytogenes are ?

Answer 25

gram-positive rods. This organism is a facultative intracellular bacteria when it infects the human host. It is unique among bacterial and fungal causes of infectious bacterial meningitis because it lacks the production of a capsule.

Question 26

This patient is experiencing cyclic fevers, night sweats, and hepatosplenomegaly after recent travel. His seizure raises suspicion for central nervous system involvement, which is confirmed on MRI showing diffuse cerebral edema. The most likely infecting agent in this case is Plasmodium falciparum and the disease is called cerebral malaria.

Four members of the Plasmodium genus of protozoa commonly infect humans. All are transmitted through the female Anopheles mosquito. Of these four species? ,

Answer 26

P. falciparum is known for its cerebral involvement, which can lead to coma and death. Of note, P. falciparum is unique among strains of malaria because it causes irregular, not cyclic, fevers. In considering diagnostic studies, blood smear may reveal trophozoite rings within RBCs

Question 27

Babesia cause babesiosis, a tick-borne illness that is geographically associated with the northeastern United States; it is less likely to present with cyclic fevers and neurologic complications.
P. malariae and P. ovale have 72-hour and 48-hour cycles, respectively. Neither is associated with cerebral malaria.
Trypanosoma brucei is the pathogen that causes African sleeping sickness. Even though the patient’s recurring fevers are consistent with African sleeping sickness, hepatosplenomegaly and seizures would be less likely.
Leishmania donovani may cause ?

Answer 27

spiking fevers and hepatosplenomegaly, but seizures and neurologic involvement are atypical.

Four Plasmodium species may cause malaria, which presents with cyclic fevers, anemia, and hepatosplenomegaly. Of the Plasmodium species that cause malaria in humans, only P. falciparum has cerebral involvement.

Question 28

This patient with an upper-respiratory-like illness of 2-week duration, who is now experiencing spontaneous coughing fits followed by emesis, presents with symptoms of second-stage pertussis, an infection caused by Bordetella pertussis.
Infection with B. pertussis, a gram-negative coccobacillus, presents in three clinical stages:

1. Catarrhal stage, lasting 1–2 weeks, characterized by symptoms that resemble an upper respiratory infection.
2. Paroxysmal stage, lasting 2–8 weeks, characterized by paroxysms of coughing, an inspiratory whoop, and posttussive vomiting.
3. Convalescent stage, in which the cough subsides over several weeks to months.
   Pertussis is seen mainly in infants and young children, but is diagnosed infrequently in the United States as a result of the nearly universal use of the pertussis vaccine. However, neither prior infection nor immunization confers lifelong immunity (although if pertussis is contracted, the illness may be less severe as a result of either of these exposures). B. pertussis can be grown on?

Answer 28

Bordet-Gengou agar and produces an exotoxin (pertussis toxin) that ADP-ribosylates and inactivates Gi proteins. Pertussis toxin leads to increased cAMP levels through unopposed activation of adenylate cyclase.

Vibrio cholerae also produces an exotoxin that leads to increased cAMP levels by ADP-ribosylating a Gs, causing a profuse watery diarrhea.

Question 29

Clostridium botulinum produces botulinum toxin that inhibits the presynaptic release of acetylcholine at the neuromuscular junction, causing flaccid paralysis. Streptococcus pyogenes produces streptolysin O that is a pore-forming toxin. Shigella dysentariae and enterohemorrhagic Escherichia coli (EHEC) encode shigatoxin, a nuclease that cleaves the ribosomal RNA of the eukaryotic 60S ribosome, shutting down protein synthesis. Corynebacterium diphtheriae produces a ?

Answer 29

diphtheria toxin, an exotoxin that ADP-ribosylates ribosomal elongation factor-2, shutting down eukaryotic protein synthesis.

Question 30

Bordetella pertussis infection begins as an upper-respiratory-like infection that progresses to paroxysms of coughing, an inspiratory whoop, and posttussive vomiting. Pertussis toxin inhibits Gi proteins which results in?

Answer 30

a net increase in cAMP levels. Vibrio cholerae similarly produces a toxin that increases cAMP levels, but results in a profuse, watery diarrhea.