A 7-month-old boy presents with recurrent pyogenic infections. Immunoglobulin levels show absent IgG, IgA, and IgM. Flow cytometry reveals mature CD4+ and CD8+ T cells are present, but B cells are completely absent. The most likely diagnosis is:

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

Explanation

X-linked agammaglobulinemia (Bruton's) is caused by Bruton tyrosine kinase (BTK) deficiency, leading to a complete block in B-cell development at the pro-B cell stage — hence absent circulating B cells but preserved T cells. Recurrent pyogenic infections appear after maternal IgG wanes at 5–7 months. CVID presents later in life with low but detectable B cells. Hyper-IgM syndrome has elevated IgM with absent IgG/IgA but CD40L deficiency, not absence of B cells. SCID has absent both T and B cells.

Reference: Ananthanarayan & Paniker's Textbook of Microbiology, 11th ed.

High-yield for: NEET PGINI-CETNExTFMGEUSMLEPLABMRCP

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