PCAT Microbiology 3

Question 1

In the complement system, which pathway is activated first upon exposure to bacterial lipopolysaccharide (LPS)?

Accepted Answer

Alternative pathway

Answer

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?

Accepted Answer

Ziehl-Neelsen stain

Answer

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?

Accepted Answer

ADP-ribosylation of Gs protein

Answer

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?

Accepted Answer

Type IV (delayed-type)

Answer

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:

Accepted Answer

Streptococcus agalactiae

Answer

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

PCAT - Pharmacy College Admission Test Practice Test

PCAT - Pharmacy College Admission Test Practice Test

PCAT Microbiology 3