USMLE Microbiology & Immunology

Question 1

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

Accepted Answer

C5-C9 (membrane attack complex)

Answer

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

Question 2

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

Accepted Answer

Tetanospasmin (blocks GABA/glycine release at inhibitory interneurons)

Answer

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

Question 3

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

Accepted Answer

TMP-SMX (trimethoprim-sulfamethoxazole)

Answer

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

Question 4

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

Accepted Answer

Hepatitis D (HDV)

Answer

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).

Question 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?

Accepted Answer

X-linked agammaglobulinemia (Bruton's)

Answer

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.

USMLE Practice Test

USMLE Practice Test

USMLE Microbiology & Immunology