Molecular Biology/Genetics USMLE Step 1 Practice Question
A newborn presents with severe lymphopenia (WBC 2,800/μL, lymphocytes 8%) discovered on routine screening after an older sibling died from Pneumocystis jirovecii pneumonia at 6 months. Vital signs are normal; temperature is 37.2°C. Flow cytometry demonstrates absent mature B and T lymphocytes with preserved natural killer cells. Chest X-ray shows no infiltrates. The infant has received no immunizations. Which of the following molecular processes is most likely defective in this infant?
Answer choices
- AExpression of CD40 ligand on activated T cells
- BClass switching from IgM to other immunoglobulin isotypes
- COxidative burst generation in phagocytes
- DPhosphorylation of tyrosine residues by BTK
- ERecombination of variable, diversity, and joining gene segmentsCorrect answer
- FCommon gamma chain 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.