An infant has severe recurrent viral, bacterial and fungal infections with oral candidiasis and Pneumocystis pneumonia. Lymph node biopsy shows no germinal centres. Flow cytometry reveals profoundly reduced CD4 T cells, normal CD8 counts, and absent HLA-DR expression on all mononuclear cells. The underlying defect is:

• A Mutation of the BTK gene preventing pre-B-cell receptor signalling
• B Failure to express MHC class II molecules due to defects in transcription factors such as CIITA ✓
• C Defective CD40 ligand on activated T cells preventing class switching
• D Absence of the common gamma chain of the IL-2 receptor

Explanation

Bare lymphocyte syndrome type II is MHC class II deficiency caused by mutations in genes controlling class II transcription such as CIITA and RFX5. Without HLA-DR, CD4 T cells cannot be positively selected in the thymus, producing a combined immunodeficiency with absent class II on all cells. BTK mutation gives X-linked agammaglobulinaemia with normal CD4 counts, CD40L defect causes hyper-IgM syndrome, and gamma chain loss is X-linked SCID without the class II finding.

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.