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Microbiology Flashcards

7 cards from real PCAT 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. In the complement system, which pathway is activated first upon exposure to bacterial lipopolysaccharide (LPS)?

    Answer: Alternative pathway

    The alternative pathway is activated spontaneously by LPS and other microbial surfaces without requiring antibodies, making it part of innate immunity.

  2. A sputum culture grows acid-fast rods. Which staining technique was used to identify these organisms?

    Answer: Ziehl-Neelsen stain

    The Ziehl-Neelsen (acid-fast) stain uses carbol fuchsin that binds mycolic acids in Mycobacteria cell walls, which resist decolorization with acid-alcohol.

  3. Which toxin mechanism causes the symptoms of cholera by permanently activating adenylyl cyclase?

    Answer: ADP-ribosylation of Gs protein

    Cholera toxin ADP-ribosylates the Gsα subunit, locking it in active form, causing constitutive adenylyl cyclase activation and massive cAMP-driven chloride secretion.

  4. Which type of hypersensitivity reaction is responsible for the tuberculin (PPD) skin test reaction?

    Answer: Type IV (delayed-type)

    The PPD reaction is a Type IV delayed hypersensitivity mediated by sensitized T cells releasing cytokines 48–72 hours after antigen exposure.

  5. A newborn develops meningitis caused by an organism that ferments lactose and is found in the mother's birth canal. The most likely pathogen is:

    Answer: Streptococcus agalactiae

    Streptococcus agalactiae (Group B Strep) colonizes the vaginal canal and is the leading cause of neonatal meningitis acquired during delivery.

  6. Which antifungal drug acts by inhibiting beta-1,3-glucan synthase, disrupting the fungal cell wall?

    Answer: Caspofungin

    Caspofungin (an echinocandin) inhibits beta-1,3-glucan synthase, which is absent in mammals, making it selectively toxic to fungi.

  7. The virulence factor that allows Streptococcus pyogenes to resist phagocytosis is:

    Answer: M protein

    M protein on the surface of S. pyogenes inhibits complement activation and opsonization, allowing the bacterium to evade phagocytosis.