A 6-month-old boy has recurrent severe bacterial infections, no tonsils, absent B cells (CD19−) on flow cytometry, and undetectable immunoglobulins. T-cell count is normal. The most likely diagnosis and affected gene are:

• A DiGeorge syndrome — TBX1 gene deletion
• B SCID — ADA gene mutation
• C Common variable immunodeficiency — TNFRSF13B mutation
• D X-linked agammaglobulinemia — BTK gene mutation ✓

Explanation

X-linked agammaglobulinemia (Bruton disease) is caused by BTK (Bruton tyrosine kinase) mutations, which block pre-B cell maturation. Boys present after 6 months (maternal IgG wanes) with absent B cells, absent immunoglobulins, absent lymphoid tissue, and recurrent encapsulated bacterial infections. DiGeorge affects T cells; SCID affects both T and B cells; CVID presents in adults with low but not absent Ig.

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

High-yield for: NEET PGINI-CETNExTFMGEUSMLEPLABMRCP

Written and medically reviewed by the StethoPrep medical team.