Immunology USMLE Step 1 Practice Question
A 2-year-old boy presents with recurrent otitis media, severe Pneumocystis jirovecii pneumonia (SpO2 88%), and chronic Cryptosporidium diarrhea. Vital signs show HR 115, RR 28, BP 95/60, temp 38.2°C. Laboratory studies reveal elevated IgM (280 mg/dL), severely low IgG (45 mg/dL), IgA (<5 mg/dL), and IgE (<2 mg/dL). B cell count is normal. He denies recent antibiotics. Which cellular interaction is most likely defective?
Answer choices
- AB7 on B cells binding CD28 on T cells
- BCD40 on B cells binding CD40 ligand on helper T cellsCorrect answer
- CFc portion of IgG binding Fc receptor on macrophages
- DMHC I on all nucleated cells binding CD8 on T cells
- EIL 5 binding eosinophils
- FICOS on T cells binding ICOSL on B cells
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