USMLE Microbiology & Immunology 1 — Questions and Answers
Question 1: A patient with recurrent Neisseria infections (meningococcal and gonococcal) has deficiency of which complement components?
- C5-C9 (membrane attack complex) (Correct answer)
- C1q, C1r, C1s
- C3
- C4
Correct answer: C5-C9 (membrane attack complex)
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?
- Tetanospasmin (blocks GABA/glycine release at inhibitory interneurons) (Correct answer)
- Tetanolysin
- Cholera toxin
- Botulinum toxin
Correct answer: Tetanospasmin (blocks GABA/glycine release at inhibitory interneurons)
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?
- TMP-SMX (trimethoprim-sulfamethoxazole) (Correct answer)
- Fluconazole
- Azithromycin
- Dapsone
Correct answer: TMP-SMX (trimethoprim-sulfamethoxazole)
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?
- Hepatitis D (HDV) (Correct answer)
- Hepatitis C (HCV)
- Hepatitis E (HEV)
- Hepatitis A (HAV)
Correct answer: Hepatitis D (HDV)
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?
- X-linked agammaglobulinemia (Bruton's) (Correct answer)
- CVID
- DiGeorge syndrome
- Severe combined immunodeficiency (SCID)
Correct answer: X-linked agammaglobulinemia (Bruton's)
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.
Question 6: Which gram-positive organism produces a toxin that causes toxic shock syndrome by acting as a superantigen?
- Staphylococcus aureus (TSST-1) (Correct answer)
- Streptococcus pyogenes (SPE)
- Clostridium perfringens
- Bacillus anthracis
Correct answer: Staphylococcus aureus (TSST-1)
S. aureus produces TSST-1, a superantigen that non-specifically activates up to 20% of T cells by cross-linking MHC II on APCs with TCR Vβ regions, causing massive cytokine release.
USMLE Microbiology & Immunology 1
A patient with recurrent Neisseria infections (meningococcal and gonococcal) has deficiency of which complement components?