A 2-year-old with recurrent infections, absent tonsils, and very low or absent serum immunoglobulins (IgG, IgA, IgM) — all four classes — is found to have a mutation in the BTK (Bruton tyrosine kinase) gene on the X chromosome. This condition is:

• A Common variable immunodeficiency (CVID)
• B Severe combined immunodeficiency (SCID)
• C X-linked agammaglobulinemia (XLA) ✓
• D Hyper-IgM syndrome

Explanation

X-linked agammaglobulinemia (XLA, Bruton agammaglobulinemia) is caused by mutations in the BTK gene, causing a block in B-cell differentiation at the pro-B cell stage. This results in absent or markedly reduced B cells and all immunoglobulin classes. Boys typically present after 6 months (when maternal immunoglobulin protection wanes) with recurrent bacterial infections (Haemophilus, Pneumococcus). Treatment is regular IVIG replacement. CVID presents later and has some residual immunoglobulins; SCID also affects T cells.

Reference: Ghai Essential Pediatrics, 10th ed.

High-yield for: NEET PGINI-CETNExTFMGEUSMLEPLABMRCP

Written and medically reviewed by the StethoPrep medical team.