A 20-year-old man presents with recurrent sinopulmonary infections, no lymphadenopathy, and absent tonsils. Peripheral blood B cells are absent; T cells are normal. Serum shows absent IgG, IgA, IgM. What is the MOST likely immunodeficiency?

• A Common variable immunodeficiency (CVID) — B cells present but fail to differentiate
• B Severe combined immunodeficiency (SCID) — absent T and B cell function
• C X-linked agammaglobulinemia (Bruton's disease) — BTK mutation blocking pre-B cell maturation ✓
• D Selective IgA deficiency — isolated IgA absent, other immunoglobulins normal

Explanation

X-linked agammaglobulinemia (XLA) is caused by mutations in Bruton's tyrosine kinase (BTK), which is essential for pre-B cell to B cell maturation. The result is an almost complete absence of mature B cells and all immunoglobulin classes, with intact T-cell numbers and function. Presentation is typically in male infants after maternal antibody wanes (after 6 months). CVID presents later with B cells present but dysfunctional. SCID affects both T and B cells.

Reference: Robbins & Cotran Pathologic Basis of Disease, 10th ed.

High-yield for: NEET PGINI-CETNExTFMGEUSMLEPLABMRCP

Written and medically reviewed by the StethoPrep medical team.