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Microbiology & Immunology Flashcards

6 cards from real USMLE practice questions. Tap to flip, then mark Knew It or Still Learning — missed cards come back until you master them.

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  1. A patient presents with rice-water stools and rapid dehydration. The causative organism produces a toxin that permanently activates adenylate cyclase. What organism is responsible?

    Answer: Vibrio cholerae

    Vibrio cholerae produces cholera toxin (CT) that ADP-ribosylates Gs-alpha, permanently activating adenylate cyclase and causing massive chloride secretion and profuse watery diarrhea.

  2. Which HIV protein integrates viral cDNA into the host genome?

    Answer: Integrase (IN)

    HIV integrase catalyzes the integration of double-stranded viral cDNA into the host cell genome, creating the provirus; it is the target of drugs like raltegravir and dolutegravir.

  3. A patient develops hemolytic anemia, thrombocytopenia, and renal failure after a bloody diarrheal illness caused by E. coli O157:H7. What toxin causes this?

    Answer: Shiga toxin (Stx)

    Shiga toxin (produced by EHEC O157:H7) inactivates the 60S ribosomal subunit by cleaving 28S rRNA, causing cell death in renal endothelium and leading to hemolytic uremic syndrome (HUS).

  4. Which organism is an obligate intracellular bacterium that causes Q fever and is transmitted through aerosolized particles from cattle or sheep?

    Answer: Coxiella burnetii

    Coxiella burnetii causes Q fever, is extremely resistant to environmental conditions, and is transmitted via inhalation of contaminated aerosols from livestock; no arthropod vector required.

  5. A patient with severe combined immunodeficiency (SCID) has absent T and B cells. ADA deficiency is found. What metabolite accumulates and is toxic to lymphocytes?

    Answer: Deoxyadenosine triphosphate (dATP)

    ADA deficiency causes accumulation of deoxyadenosine, which is phosphorylated to toxic dATP in lymphocytes, inhibiting ribonucleotide reductase and blocking DNA synthesis.

  6. Which gram-positive rod forms spores, is found in soil, and causes gas gangrene by producing a lecithinase (alpha-toxin)?

    Answer: Clostridium perfringens

    C. perfringens produces alpha-toxin (lecithinase/phospholipase C), which hydrolyzes membrane phospholipids, causing myonecrosis (gas gangrene) and hemolysis.