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

C1q, C1r, C1s

Answer

C3

Answer

C4

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

Tetanolysin

Answer

Cholera toxin

Answer

Botulinum toxin

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

Fluconazole

Answer

Azithromycin

Answer

Dapsone

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 C (HCV)

Answer

Hepatitis E (HEV)

Answer

Hepatitis A (HAV)

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

CVID

Answer

DiGeorge syndrome

Answer

Severe combined immunodeficiency (SCID)

Question 6

Which gram-positive organism produces a toxin that causes toxic shock syndrome by acting as a superantigen?

Accepted Answer

Staphylococcus aureus (TSST-1)

Answer

Streptococcus pyogenes (SPE)

Answer

Clostridium perfringens

Answer

Bacillus anthracis