UWorld Microbiology and Immunology

Question 1

A Gram-positive coccus in clusters is catalase-positive and coagulase-positive. What is the most likely organism?

Accepted Answer

Staphylococcus aureus

Answer

Staphylococci grow in clusters and are catalase-positive. Among them, S. aureus is the one that is coagulase-positive. S. epidermidis is coagulase-negative, and streptococci and enterococci are catalase-negative and grow in chains.

Question 2

The tuberculin skin (PPD) test is an example of which type of hypersensitivity reaction?

Accepted Answer

Type IV

Answer

The PPD test is a delayed-type (type IV) hypersensitivity reaction mediated by sensitized T cells, not antibodies. Induration peaks at 48–72 hours.

Question 3

What is the most common bacterial cause of community-acquired pneumonia in adults?

Accepted Answer

Streptococcus pneumoniae

Answer

Streptococcus pneumoniae, a lancet-shaped Gram-positive diplococcus, is the most common cause of typical community-acquired pneumonia in adults.

Question 4

A 9-month-old boy has recurrent bacterial infections, absent tonsils, and very low levels of all immunoglobulins. Which defect is most likely?

Accepted Answer

Defective BTK tyrosine kinase

Answer

X-linked (Bruton) agammaglobulinemia is caused by a BTK mutation that blocks B-cell maturation. The result is absent B cells, absent lymphoid tissue such as tonsils, and very low immunoglobulins. Symptoms start after maternal IgG wanes, at around 6 months.

Question 5

A traveler develops profuse 'rice-water' diarrhea. How does the responsible toxin work?

Accepted Answer

ADP-ribosylates Gs, raising cAMP

Answer

Cholera toxin from Vibrio cholerae permanently activates Gs by ADP-ribosylation. The resulting rise in cAMP drives chloride and water secretion into the gut lumen. Pertussis toxin inactivates Gi. Shiga toxin inactivates the 60S subunit.

UWorld Practice Test

UWorld Practice Test

UWorld Microbiology and Immunology