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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 with recurrent Neisseria infections (meningococcal and gonococcal) has deficiency of which complement components?

    Answer: C5-C9 (membrane attack complex)

    Deficiency in terminal complement components C5-C9 (membrane attack complex) prevents lysis of encapsulated gram-negative diplococci, predisposing to recurrent Neisseria infections.

  2. Which bacterial toxin is responsible for the classic symptoms of tetanus by blocking inhibitory neurotransmitter release?

    Answer: Tetanospasmin (blocks GABA/glycine release at inhibitory interneurons)

    Tetanospasmin cleaves synaptobrevin (VAMP) in inhibitory interneurons of the spinal cord, blocking GABA and glycine release, causing spastic paralysis.

  3. A patient with AIDS develops Pneumocystis jirovecii pneumonia. What prophylaxis should have been started when CD4 count fell below 200 cells/μL?

    Answer: TMP-SMX (trimethoprim-sulfamethoxazole)

    TMP-SMX is the first-line prophylaxis for PCP when CD4 count falls below 200 cells/μL in HIV-infected patients.

  4. Which hepatitis virus uses RNA-dependent RNA polymerase and requires co-infection with hepatitis B virus for replication?

    Answer: Hepatitis D (HDV)

    Hepatitis D (delta virus) is a defective RNA virus that requires HBV surface antigen (HBsAg) for encapsidation, and can only infect HBV-positive patients (co- or superinfection).

  5. A 5-year-old child has recurrent bacterial infections since 6 months of age, absent B cells, and very low immunoglobulin levels. The cellular immune system is intact. What is the diagnosis?

    Answer: X-linked agammaglobulinemia (Bruton's)

    Bruton's agammaglobulinemia is an X-linked disorder caused by BTK mutation, preventing B cell maturation; patients have absent B cells, low IgG/IgA/IgM, and intact T cell immunity.

  6. Which gram-positive organism produces a toxin that causes toxic shock syndrome by acting as a superantigen?

    Answer: Staphylococcus aureus (TSST-1)

    S. aureus produces TSST-1, a superantigen that non-specifically activates up to 20% of T cells by cross-linking MHC II on APCs with TCR Vβ regions, causing massive cytokine release.