PCAT Microbiology 3 — Questions and Answers
Question 1: In the complement system, which pathway is activated first upon exposure to bacterial lipopolysaccharide (LPS)?
- Classical pathway
- Lectin pathway
- Alternative pathway (Correct answer)
- Antibody-dependent pathway
Correct answer: Alternative pathway
The alternative pathway is activated spontaneously by LPS and other microbial surfaces without requiring antibodies, making it part of innate immunity.
Question 2: A sputum culture grows acid-fast rods. Which staining technique was used to identify these organisms?
- Gram stain
- India ink preparation
- Ziehl-Neelsen stain (Correct answer)
- Giemsa stain
Correct 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.
Question 3: Which toxin mechanism causes the symptoms of cholera by permanently activating adenylyl cyclase?
- ADP-ribosylation of Gs protein (Correct answer)
- Inhibition of protein synthesis
- Pore formation in cell membranes
- Activation of phospholipase C
Correct 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.
Question 4: Which type of hypersensitivity reaction is responsible for the tuberculin (PPD) skin test reaction?
- Type I (IgE-mediated)
- Type II (cytotoxic)
- Type III (immune complex)
- Type IV (delayed-type) (Correct answer)
Correct 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.
Question 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:
- Listeria monocytogenes
- Neisseria meningitidis
- Streptococcus agalactiae (Correct answer)
- Escherichia coli K1
Correct answer: Streptococcus agalactiae
Streptococcus agalactiae (Group B Strep) colonizes the vaginal canal and is the leading cause of neonatal meningitis acquired during delivery.
Question 6: Which antifungal drug acts by inhibiting beta-1,3-glucan synthase, disrupting the fungal cell wall?
- Fluconazole
- Amphotericin B
- Caspofungin (Correct answer)
- Flucytosine
Correct answer: Caspofungin
Caspofungin (an echinocandin) inhibits beta-1,3-glucan synthase, which is absent in mammals, making it selectively toxic to fungi.
Question 7: The virulence factor that allows Streptococcus pyogenes to resist phagocytosis is:
- Streptolysin O
- Hyaluronidase
- M protein (Correct answer)
- Erythrogenic toxin
Correct answer: M protein
M protein on the surface of S. pyogenes inhibits complement activation and opsonization, allowing the bacterium to evade phagocytosis.
PCAT Microbiology 3
In the complement system, which pathway is activated first upon exposure to bacterial lipopolysaccharide (LPS)?