A 6-month-old infant has severe recurrent viral, fungal, and bacterial infections. Flow cytometry shows absent CD4+ T cells, normal numbers of CD8+ T cells and B cells, and very low immunoglobulin levels. The defect prevents expression of HLA-DP, HLA-DQ, and HLA-DR on all cells. The most likely diagnosis is:

• A MHC class II deficiency (bare lymphocyte syndrome) ✓
• B DiGeorge syndrome
• C ZAP-70 deficiency
• D Reticular dysgenesis

Explanation

Bare lymphocyte syndrome type II is caused by mutations in transcription factors controlling MHC class II genes (such as CIITA or RFX proteins), abolishing HLA-DP, DQ, and DR expression. Without class II molecules, CD4+ T cells cannot be positively selected in the thymus, producing profound CD4 deficiency with impaired helper function despite normal CD8 and A cell counts. ZAP-70 deficiency blocks CD8 development as well, and DiGeorge syndrome reduces both T cell subsets through thymic aplasia.

Reference: Robbins and Cotran Pathologic Basis of Disease, 10th ed.

High-yield for: NEET PGINI-CETNExTFMGEUSMLEPLABMRCP

Written and medically reviewed by the StethoPrep medical team.