A 4-year-old boy has recurrent sinopulmonary infections, absent tonsils and lymph nodes, markedly low immunoglobulins (IgG, IgA, IgM), and absent B cells (CD19+ cells 0%) on flow cytometry. Maternal grandfather had similar disease. What is the inheritance pattern, and what gene is mutated?

• A Autosomal recessive; RAG1/RAG2 mutation
• B Autosomal dominant; STAT3 mutation
• C X-linked recessive; IL2RG (common gamma chain) mutation
• D X-linked recessive; BTK (Bruton tyrosine kinase) mutation ✓

Explanation

X-linked agammaglobulinemia (XLA, Bruton disease) is caused by mutations in the BTK gene on chromosome Xq21.3, encoding Bruton tyrosine kinase, which is essential for pre-B cell to B cell maturation. Affected males have absent B cells, absent immunoglobulins, and absent/small lymphoid tissue (no tonsils, no lymph nodes). The maternal grandfather having the disease is consistent with X-linked inheritance (grandfather → carrier daughter → affected grandson). Treatment is lifelong intravenous immunoglobulin (IVIG) replacement. RAG1/RAG2 mutations cause autosomal recessive SCID with absent T and B cells.

Reference: Ghai Essential Pediatrics, 10th ed.

High-yield for: NEET PGINI-CETNExTFMGEUSMLEPLABMRCP

Written and medically reviewed by the StethoPrep medical team.