MD Infectious Disease and Microbiology 5

Question 1

A 28-year-old traveler to Mexico develops profuse watery 'rice-water' diarrhea with no fecal leukocytes. The causative organism produces a toxin that permanently activates Gs protein by ADP-ribosylation. Which organism?

Accepted Answer

Vibrio cholerae

Answer

Vibrio cholerae produces cholera toxin, which ADP-ribosylates the Gs alpha subunit, locking adenylyl cyclase in the 'on' position, causing massive cAMP-driven chloride and water secretion.

Question 2

A 50-year-old gardener develops a nodular lesion on his hand that spreads up the lymphatics in a linear pattern after a thorn injury. KOH prep shows cigar-shaped yeast at 37°C. Which antifungal is preferred?

Accepted Answer

Itraconazole

Answer

Sporotrichosis (Sporothrix schenckii) presenting with lymphocutaneous spread is treated with itraconazole; potassium iodide is an alternative for cutaneous disease.

Question 3

A 40-year-old develops ascending flaccid paralysis starting in both legs 2 weeks after a GI illness with Campylobacter jejuni. CSF shows elevated protein with normal cell count (albuminocytologic dissociation). Which condition is this?

Accepted Answer

Guillain-Barré syndrome

Answer

Guillain-Barré syndrome is a post-infectious demyelinating polyneuropathy often triggered by C. jejuni due to molecular mimicry between gangliosides and bacterial lipooligosaccharide; CSF shows albuminocytologic dissociation.

Question 4

A 60-year-old construction worker in the Southwest US develops fever, chest pain, eosinophilia, and a coin lesion on CXR after exposure to soil dust. Serology is positive. Which organism is responsible?

Accepted Answer

Coccidioides immitis

Answer

Coccidioides immitis is endemic to the arid Southwest US (San Joaquin Valley fever) and causes pulmonary infection with eosinophilia; large endosporulating spherules are the tissue form.

Question 5

An HIV-positive patient with CD4 count of 150 cells/μL develops ring-enhancing brain lesions on MRI with surrounding edema. Serology is positive for IgG antibodies. What is the most likely diagnosis and empiric treatment?

Accepted Answer

Cerebral toxoplasmosis; pyrimethamine + sulfadiazine

Answer

Multiple ring-enhancing lesions in an HIV patient with positive Toxoplasma IgG should be treated empirically with pyrimethamine + sulfadiazine (+ leucovorin); response within 2 weeks confirms diagnosis.

MD - Doctor of Medicine Practice Test

MD - Doctor of Medicine Practice Test

MD Infectious Disease and Microbiology 5