Micro USMLE 8-21 (5)

Question 1

A biotechnology firm is developing a new, small-protein drug designed to prevent the spread of a sexually transmitted infection. Scientists want to block the infectious step of the bacteria’s reproductive cycle. In a patient infected with the targeted pathogen, a Giemsa-stained smear of urethral discharge would look similar to the image below.During which stage of this pathogen’s life cycle is it most infectious?

Answer 1

Extracellular elementary body
Giemsa stain showing intracytoplasmic inclusions in the context of a urethral discharge, potentially due to a sexually transmitted infection, is indicative of Chlamydia trachomatis. Cytoplasmic inclusions can be seen on Giemsa- or fluorescent antibody–stained urethral or cervical smear, but diagnosis of Chlamydia can also be made from a urine sample using nucleic acid amplification techniques. Although it is frequently asymptomatic, C. trachomatis infection can cause urethritis, cervicitis, and pelvic inflammatory disease (PID) in women, as well as conjunctivitis and reactive arthritis (Reiter syndrome). Treatment of Chlamydia infection requires a course of either doxycycline or azithromycin. Chlamydia is infectious when it reaches the developmental stage described as an extracellular elementary body, as this form can attach and enter host cells.

Question 2

he cytoplasmic inclusions (arrows in image above) are non-infectious as are the intracellular elementary bodies. Similarly, independent reticulate bodies and reticulate bodies that are?

Answer 2

in the process of multiplying are both intracellular, and therefore are not infectious. The stages of Chlamydia development and relationship to infectivity are explained by the figure and table below.

Question 3

Chlamydia trachomatis, which can cause urethritis, cervicitis, and pelvic inflammatory disease in women, is infectious when it is as an elementary body that can enter host cells from ?

Answer 3

the extracellular domain. While all of the intracellular forms of C. trachomatis are necessary for the reproduction of the organism, only the extracellular form is considered infectious.

Question 4

This woman presents with a 1- to 2-week history of fatigue, cold intolerance, constipation and neck pain following a likely viral illness. Her examination is significant for a tender thyroid gland. Signs of hypothyroidism with a tender thyroid are most likely to be related to De Quervain thyroiditis, a common cause of transient hypothyroidism seen after viral illnesses. A hyperthyroid stage caused by unregulated T4 and T3 release generally precedes the transient hypothyroid stage.
De Quervain thyroiditis, also known as?

Answer 4

subacute granulomatous thyroiditis, is characterized by an exquisitely tender thyroid gland, elevated erythrocyte sedimentation rate, and other manifestations of hypothyroidism (eg, fatigue, cold intolerance, cool skin, decreased deep tendon reflexes) seen in this patient. Because patients quickly return to a euthyroid state, no treatment is necessary.

Question 5

Numerous viral agents such as mumps, Epstein-Barr virus, coxsackie virus, flu, and adenovirus have been implicated in De Quervain thyroiditis but the precise cause of the condition is unknown. Graves disease is an autoimmune form of hyperthyroidism caused by ?

Answer 5

production of autoantibodies that activate the thyroid-stimulating hormone receptor. Hashimoto thyroiditis is characterized by antimicrosomal and antithyroglobulin antibodies that lead to hypothyroidism; however, this autoimmune condition typically has a slow course and presents with a nontender thyroid. Postpartum thyroiditis is associated with painless hypothyroidism and would be seen 1 year after pregnancy. Vitamin B12 deficiency leads to a macrocytic anemia and peripheral neuropathy and does not cause elevated thyroid-stimulating hormone levels or thyroid gland tenderness.

Question 6

De Quervain thyroiditis is a transient hypothyroidism seen after viral illnesses. It is characterized by an exquisitely tender thyroid gland, elevated erythrocyte sedimentation rate, and other manifestations of ?

Answer 6

hypothyroidism, including fatigue, cold intolerance, cool skin, and decreased deep tendon reflexes.

Question 7

The patient’s presentation is representative of the second stage of subacute sclerosing panencephalitis (SSPE), a sequela of infection with rubeola, or measles virus. SSPE typically presents 7–10 years after initial infection with the virus and results from the failure of mature viral particles to form. This action leads to persistent replication of the virus and nonproductive infection, eventually resulting in demyelination of CNS neurons.
The initial stage of the condition presents with dementia and personality changes and lasts up to 1 year. The second stage of the condition is characterized by severe myoclonus and typically lasts for 3–12 months. The third and fourth stages are characterized by?

Answer 7

worsening dementia and deterioration of the autonomic nervous system, as well as decorticate rigidity or flaccidity. The condition is typically fatal. A diagnosis can be made by the detection of oligoclonal bands in the CSF. High titers of measles antibodies are also often present in the serum of affected patients.

Question 8

The fact that the patient is from rural India and is unsure of her vaccination status suggests that she may not have received all of her childhood immunizations, including her measles vaccines.

JC virus causes progressive multifocal leukoencephalopathy and manifests in altered mental status, motor deficits, and visual symptoms. It rarely affects immunocompetent persons. Poliovirus infection is characterized by flaccid paralysis and muscular atrophy. Rabies virus spreads from the site of the initial bite from an infected animal to motor and sensory neurons and can lead to ?

Answer 8

convulsions, paresthesias, cognitive effects, and flaccid paralysis; the time course of this patient’s illness is extremely slow for rabies. The rubella virus, if acquired antenatally, can lead to congenital rubella syndrome, which is characterized by ophthalmologic and cardiac defects and fetal mortality.

Question 9

Subacute sclerosing panencephalitis (SSPE) is a sequela of rubeola, or measles virus, and can be detected by the presence of oligoclonal bands in CSF. High titers of measles antibodies are often present in the serum of affected patients. Be suspicious of SSPE in a patient who has an unknown vaccination history, is from an endemic area, and presents with ?

Answer 9

myoclonus and worsening dementia.

Question 10

The patient received a renal transplant 10 years ago and takes immunosuppressant drugs as a result. When he is not working, he likes to garden or take his children to a local park, activities that would put him in regular contact with soil and pigeon droppings, and therefore with Cryptococcus neoformans.
In immunosuppressed individuals, C. neoformans can lead to cryptococcal pneumonia. This is the second most common clinical manifestation of cryptococcal infection. Symptoms can include fever, fatigue, cough, and pleuritic chest pain. Other less common findings may include ?

Answer 10

hemoptysis, auscultatory rales, or a pleural rub. C. neoformans is visualized via India ink stain and detected by serum cryptococcal antigen.

Question 11

Board review guides often emphasize that this species is a causative agent of fungal meningitis in immunocompromised patients (with Blastomyces, Coccidioides, and Histoplasma as causes of fungal pneumonia). Students should note, however, that these are oversimplifications. All the systemic mycoses can cause infection in a number of sites. Although meningoencephalitis is the most common clinical manifestation of cryptococcal disease, it is not the only one. In fact, the initial site of inoculation with Cryptococcus is? t

Answer 11

he lung; meningitis can ensue only after the yeasts have disseminated from this site. Patients present more often with cryptococcal meningitis, in large part because many cryptococcal lung infections are asymptomatic.

Other organs, such as the abdomen, bones, skin, and genitourinary tract, may be affected in disseminated disease; however, these are not the initial site of infection and are therefore rarer manifestations of cryptococcal infection.

Question 12

This means that diffuse abdominal pain with rebound tenderness is not likely to found in this patient. Peritonitis is a rare manifestation of infection with C. neoformans. Likewise, dull bone pain that worsens on movement is not a symptom that this patient would likely experience. Osteolytic bone lesions are? another less common manifestation of disseminated cryptococcal infection. Clinical presentation can vary (acute or chronic, and generally nonspecific occur symptoms first); however, bony involvement is not likely in the initial presentation of the disease.

Cutaneous lesions are found in 10%–15% of cases of disseminated disease involving C. neoformans, and they are often signs of ?

Answer 12

a particularly severe disease. Although the patient is immunosuppressed, it is unlikely that the microorganism was disseminated from the initial infection site in the lung.

And finally prostatitis can occur in cases of disseminated cryptococcal infection. It is seen occasionally in cryptococcal pneumonia. But it is unlikely that this patient would first present with symptoms of prostatitis (urinary frequency and urgency).

Question 13

Cryptococcus neoformans, visualized via India ink stain and detected by serum cryptococcal antigen, is carried by pigeons. The most common disease caused by?

Answer 13

this microorganism is meningoencephalitis, followed by cryptococcal pneumonia

Question 14

The child presents with asymptomatic, flesh-colored papules that show central umbilication, which is characteristic of molluscum contagiosum. Molluscum contagiosum is a member of the poxvirus family that causes a localized infection consisting of nonerythematous, pearly, dome-shaped papules on the skin. However, this infection frequently leads to localized dermatitis, causing the surrounding skin to become pink, dry, and pruritic. As the papules resolve, the skin may become inflamed and crust over for a week or two.
Molluscum contagiosum is a common disease of childhood and often spreads via?

Answer 14

direct skin-to-skin contact or via indirect contact such as with the sharing of bath towels. An association with use of swimming pools has been reported and is likely where this patient became infected. The lesions may appear anywhere on the body except the palms and soles, and are usually self-limited in immunocompetent people. If the lesions appear on the genitalia of sexually active people, it is considered a sexually transmitted disease. In immunocompromised patients, particularly those with HIV-positive status, lesions can be quite large (eg, giant molluscum) and widespread.

Question 15

Molluscum contagiosum is not caused by adenovirus, hepadnavirus, herpesvirus, papillomavirus, or polyomavirus. Adenovirus commonly presents with pharyngitis and coryza, and hepadnavirus causes hepatitis B. Herpesviruses are responsible for genital herpes and cold sores (HSV-1 and -2) as well as chickenpox and shingles (varicella zoster). Papillomavirus causes ?

Answer 15

the common wart in children, and reactivation of the polyomavirus results in progressive multifocal leukoencephalopathy (PML) in immunosuppressed patients.

Question 16

Molluscum contagiosum is caused by a poxvirus and is characterized by pink papules, which may be umbilicated. Itching may be present or absent. Molluscum contagiosum is commonly transmitted by skin-skin contact in children, but is associated with ?

Answer 16

sexual transmission in adults.

Question 17

The syndrome described in the vignette is consistent with Guillain-Barré syndrome (GBS), which is a common cause of acute peripheral neuropathy. Two-thirds of patients have preceding viral or gastrointestinal illness. Campylobacter jejuni, a curved, oxidase-positive, gram-negative bacterium that causes gastroenteritis, has been associated with GBS. The pathogenesis is thought to involve generation of antiganglioside antibodies induced by C. jejuni exposure.
GBS manifests as?

Answer 17

an ascending paralysis with diminished reflexes and progresses over a period of days. Findings include elevated γ-globulin, decreased nerve conduction velocity indicative of demyelination, and albuminocytologic dissociation in the CSF (increased protein, normal WBC). Nerve biopsy (which is rarely done) would show segmental demyelination with an endoneurial inflammatory infiltration.

Question 18

Streptococcus pneumoniae (α-hemolytic, encapsulated, gram-positive cocci that produces an IgA protease) that results in bacteremia, meningitis, osteomyelitis, or septic arthritis
Pseudomonas aeruginosa (non-lactose-fermenting, oxidase-positive, gram-negative, aerobic bacilli) that causes otitis externa, urinary tract infection, pneumonia, and sepsis in immunocompromised hosts
Clostridium botulinum (rod-shaped, gram-positive, spore-forming anaerobe that produces a heat-labile toxin) inhibits ?

Answer 18

acetylcholine release at the neuromuscular junction and causes flaccid paralysis
Treponema pallidum (spiral-shaped bacteria with axial filaments) is the cause of syphilis and causes tabes dorsalis, which manifests as loss of fine touch and proprioception

Question 19

Guillain-Barré syndrome may be preceded by Campylobacter jejuni infection and is characterized by an ascending paralysis that evolves ?

Answer 19

over days to weeks with the loss of deep tendon reflexes. CSF analysis shows albuminocytologic dissociation, elevated γ-globulin, and decreased nerve conduction velocity indicative of demyelination.

Question 20

This patient presents with nausea, abdominal cramping, bloating, and watery diarrhea. Although a number of bacterial and viral infections can manifest with such symptoms, the patient’s recent camping trip suggests that he has been infected with Giardia lamblia. Colonization of the gut by Giardia trophozoites results in small bowel inflammation and villous atrophy, which reduces absorptive capability. The image of the duodenal aspirate reveals?

Answer 20

a pear-shaped trophozoite.
Giardia may be diagnosed by microscopic detection of the organism in stool, antigen detection, or nucleic acid amplification. Trophozoites have four pairs of flagella and two nuclei. They attach to epithelial cells in duodenal and jejunal crypts through a sucking disk on their ventral surface. The trophozoites absorb nutrients from the host and cause inflammation and malabsorption, but they do not invade the intestinal cells.

Question 21

Activation of enteral adenylate cyclase is associated with bacterial organisms and is likely to cause a secretory diarrhea, which is a separate mechanism from that of diarrhea caused by Giardia lamblia. In addition, enzyme depletion that can occur at the intestinal brush border could lead to reduced digestion and absorption, seen in conditions such as lactose intolerance. Invasion of the intestinal lining is more likely to manifest as a ?

Answer 21

bloody diarrhea, and viral infection of the cells of the small intestinal villi would not result in the presence of a microorganism in the duodenal aspirate.

Question 22

Giardia trophozoites can lead to small bowel inflammation and villous atrophy, resulting in ?

Answer 22

reduced absorptive capability and malabsorption. Giardia infection is diagnosed by duodenal aspiration.

Question 23

This patient presents with a nonproductive cough, low-grade fever, and malaise. These symptoms, in association with patchy areas of congestion without consolidation, are suggestive of atypical pneumonia.
In patients who present with insidious onset of dry cough, low-grade fever, headache, myalgias, and nausea or emesis, an atypical pneumonia should be considered. X-ray of the chest is often more impressive than clinical examination findings and is characterized by a patchy interstitial infiltrate (see image). Treatment consists of ?

Answer 23

antibiotic therapy with a macrolide, usually azithromycin, for 5 days.

Mycoplasma pneumoniae is the most common cause of atypical pneumonia. Alhough Mycoplasma pneumonia is largely a clinical diagnosis, multiplex polymerase chain reaction (PCR) from a nasopharyngeal sample can be used to detect Mycoplasma and is now the diagnostic test of choice for suspected Mycoplasma infections because of its high specificity and sensitivity.

Question 24

Eosinophil cationic protein (ECP) levels are nonspecific in the diagnosis of Mycoplasma pneumonia, since ECP can also be elevated in asthma. Giemsa stain is the gold standard for diagnosis of malaria, not Mycoplasma pneumonia. Mycoplasma is pleomorphic and contains sterols, which do not Gram stain. Mycoplasma can be detected by the cold agglutinin test, but this is not the preferred method of detection because it has? .

Answer 24

low sensitivity and specificity; PCR is much more reliable

Question 25

Atypical pneumonia can present with a nonproductive cough, low-grade fever, and malaise. Patients with atypical pneumonia also have patchy areas of congestion but no consolidation in the lungs. Mycoplasma pneumoniae is a common cause of atypical pneumonia and can be detected by?

Answer 25

means of multiplex polymerase chain reaction (PCR) from a nasopharyngeal sample.

Question 26

The question describes infection by Corynebacterium diphtheriae. This child is particularly at risk because she has not received the appropriate immunization (the diphtheria-pertussis-tetanus vaccine). Classically, C. diphtheriae infection produces a sore throat, difficulty swallowing, swollen neck, and fever. Coinfection with group A strep is sometimes observed. Other sequelae include myocarditis and peripheral nerve palsies. Physical examination may reveal the presence of a grayish pseudomembrane in the pharynx. C. diphtheriae’s virulence factor, a two-subunit exotoxin responsible for these symptoms, is derived from?

Answer 26

a temperate bacteriophage. Bacteriophages are viruses that only infect bacteria. Once a bacteriophage infects a host, it may either replicate immediately and lyse the bacterium (a lytic bacteriophage), or incorporate its DNA into the bacterial genome and enter a proviral latent state (a temperate bacteriophage). Certain temperate bacteriophages carry genes that transform the phenotype of the host once they become incorporated. This is known as lysogenic conversion. Only strains of C. diphtheriae that have been lysogenized by a phage encoding an exotoxin are capable of causing diphtheria. Other examples of lysogenic conversion include strains of Streptococcus pyogenes, the group A streptococcal species that causes scarlet fever, and strains of Clostridium botulinum, the gram-positive bacillus that causes botulism.

Question 27

The pathogenesis of certain bacteria is dependent on the expression of a temperate bacteriophage exotoxin, a process known as lysogenic conversion. Examples include?

Answer 27

diphtheria (Corynebacterium diphtheriae), scarlet fever (Streptococcus pyogenes), botulism (Clostridium botulinum), and cholera (Vibrio cholerae).

Question 28

Diabetics are more susceptible to mucormycosis than are members of the general population. It is thought that the high glucose content in their nasal secretions is associated with increased vulnerability. With Mucor having been identified in the patient’s nasal sinuses, the main medical concern here is the possible introduction of the microorganism into the meninges, which can cause ?

Answer 28

life-threatening meningitis. Even if the patient does not have meningitis, the infection must still be treated emergently to prevent damage to facial structures and nerves.

Mucor infection can cause life-threatening meningitis in patients with diabetes.

Question 29

The patient arrives with a mix of symptoms that might be taken lightly in someone his age, including low fever, malaise, headache, and stiff neck. However, the combination of his physical exam and the cerebrospinal fluid (CSF) laboratory values indicate that he likely has bacterial meningitis. Bacterial meningitis is characterized by increased polymorphonuclear leukocytes, increased protein, and decreased glucose in the CSF. The most likely cause of bacterial meningitis in adults is Streptococcus pneumoniae (lancet-shaped diplococci), followed by Neisseria meningitidis. In teenagers, however, N. meningitidis must be considered. S. pneumoniae is a gram-positive diplococcus, whereas N. meningitidis is a gram-negative diplococcus that ferments maltose and glucose.
Individuals living in close quarters (eg, soldiers, college students) are at increased risk of N. meningitidis infection. Often a petechial rash (like that shown in the image here) will occur when meningococcemia occurs in conjunction with meningitis. Meningococcemia can also lead to ?

Answer 29

Waterhouse-Friederichsen syndrome, which is infarction of the adrenal gland causing adrenal insufficiency. N. meningitidis should be cultured on Thayer-Martin media, and antibiotic therapy with ceftriaxone should be initiated as soon as there is suspicion of a bacterial cause. Close contacts of the patient should be given chemoprophylaxis, typically with ceftriaxone or rifampin.

Question 30

A gram-negative bacilli refers to E. coli, the second most common cause of meningitis in neonates (the first being group B streptococci).
A gram-negative coccobacilli refers to Haemophilus influenzae.
Gram-positive bacilli refers to Clostridium, Corynebacterium, Listeria, and Bacillus. Listeria monocytogenes is the third most common cause of meningitis in newborns.
Viral meningitis characteristically results in ?

Answer 30

lymphocytosis, not increased polymorphonuclear leukocytes, and glucose in the CSF is typically normal.

The most likely cause of meningitis in the population is Streptococcus pneumoniae, a lancet-shaped diplococcus. However, in young adults, especially those living in close quarters who present with a petechial rash, an important pathogen to consider is Neisseria meningitidis, a gram-negative diplococcus.