A 6-month-old girl has severe recurrent infections with Pneumocystis jirovecii, Cryptosporidium diarrhea, and herpesviruses. Lymphocyte count is normal. Flow cytometry shows normal CD8 and CD20 populations but virtually absent CD4 T cells, and lymphocytes fail to stimulate in mixed lymphocyte culture. The most likely diagnosis is:

• A X-linked agammaglobulinemia due to BTK mutation
• B DiGeorge syndrome with thymic aplasia
• C Wiskott-Aldrich syndrome due to WASP mutation
• D Bare lymphocyte syndrome due to MHC class II deficiency ✓

Explanation

MHC class II deficiency (bare lymphocyte syndrome, CIITA or RFX genes) prevents thymic selection of CD4 T cells, so CD4 numbers are profoundly low despite normal total lymphocytes, CD8 cells are preserved, and CD4 responses cannot be mounted. Normal CD8 and B-cell counts exclude SCID variants and XLA, and the absence of conotruncal defects, hypocalcemia, or eczema-thrombocytopenia excludes DiGeorge and Wiskott-Aldrich syndromes.

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.