Immunology USMLE Step 1 Practice Question
A 6-year-old boy presents with recurrent sinopulmonary infections and chronic ulcerating skin lesions caused by respiratory viruses over the past 2 years. Vital signs show HR 102/min, RR 24/min, temp 37.2°C, SpO2 92% on room air. Flow cytometry reveals CD8+ T cells 8% (normal 15-25%), CD4+ T cells 32% (normal), and normal immunoglobulin levels. Chest X-ray shows bronchial wall thickening without pneumonia. A defect in which of the following is most likely present?
Answer choices
- ABTK required for B cell maturation
- BCD40 ligand on activated helper T cells
- CTAP transporter required for peptide loading onto MHC ICorrect answer
- DC5 through C9 complement proteins
- EAIRE protein required for thymic negative selection
- FRFX5 deficiency causing absent MHC class II expression
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