Immunology USMLE Step 1 Practice Question
A 3-year-old boy who received BCG vaccine abroad develops disseminated infection with fever to 38.5°C, HR 108/min, and hepatosplenomegaly. He subsequently presents with recurrent Salmonella bacteremia (blood culture positive). Laboratory evaluation shows normal lymphocyte count of 4,500/μL and normal immunoglobulin levels. He has no history of recurrent pyogenic infections. Which immune defect most likely explains his susceptibility to intracellular pathogens?
Answer choices
- ADefective terminal complement formation
- BDefective IL 12 signaling leading to impaired Th1 differentiationCorrect answer
- CDefective class switching due to CD40 ligand deficiency
- DDefective C1 esterase inhibitor
- EDefective B cell maturation due to BTK mutation
- FIFN-gamma receptor deficiency
See the full explanation
Get the correct-answer rationale, why each distractor is wrong, the underlying mechanism, and high-yield associations — plus adaptive practice that targets your weak areas — with a free MedBoardPRO account.