MD Infectious Disease and Microbiology 5 — Questions and Answers
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?
- Enterotoxigenic E. coli
- Vibrio cholerae (Correct answer)
- Salmonella typhi
- Campylobacter jejuni
Correct answer: Vibrio cholerae
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?
- Fluconazole
- Itraconazole (Correct answer)
- Amphotericin B
- Griseofulvin
Correct answer: Itraconazole
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?
- Botulism
- West Nile virus encephalitis
- Guillain-Barré syndrome (Correct answer)
- Transverse myelitis
Correct answer: Guillain-Barré syndrome
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?
- Blastomyces dermatitidis
- Histoplasma capsulatum
- Coccidioides immitis (Correct answer)
- Aspergillus fumigatus
Correct answer: Coccidioides immitis
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/ÎĽL 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?
- CNS lymphoma; methotrexate
- Cryptococcal meningitis; fluconazole
- Cerebral toxoplasmosis; pyrimethamine + sulfadiazine (Correct answer)
- Progressive multifocal leukoencephalopathy; antiretrovirals
Correct answer: Cerebral toxoplasmosis; pyrimethamine + sulfadiazine
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.
Question 6: A 25-year-old presents with painless cervical lymphadenopathy and night sweats after a cat scratch. Warthin-Starry silver stain of a lymph node biopsy shows pleomorphic gram-negative rods. Which organism is responsible?
- Francisella tularensis
- Bartonella henselae (Correct answer)
- Yersinia pestis
- Pasteurella multocida
Correct answer: Bartonella henselae
Cat-scratch disease is caused by Bartonella henselae, transmitted by cat scratch or bite, producing regional lymphadenopathy; Warthin-Starry silver stain identifies the organisms in tissue.
Question 7: A 35-year-old develops a painful vesicular rash in a dermatomal distribution on the left thorax that does not cross the midline. Which antiviral reduces duration and severity if started within 72 hours of rash onset?
- Acyclovir or valacyclovir (Correct answer)
- Ganciclovir
- Foscarnet
- Ribavirin
Correct answer: Acyclovir or valacyclovir
Herpes zoster (VZV reactivation) manifests as a painful dermatomal vesicular rash; oral acyclovir or valacyclovir initiated within 72 hours reduces duration, severity, and risk of postherpetic neuralgia.
MD Infectious Disease and Microbiology 5
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?