A 6-month-old male infant has recurrent bacterial and fungal infections, absent lymph nodes and tonsils, and no thymic shadow on chest X-ray. Lymphocyte count is markedly low with absent T, B, and NK cells. The most likely diagnosis is:

• A X-linked agammaglobulinemia (Bruton's disease)
• B DiGeorge syndrome
• C Common variable immunodeficiency (CVID)
• D Severe combined immunodeficiency (SCID) — most likely X-linked (gamma-c chain deficiency) ✓

Explanation

The absence of all lymphocyte lineages (T, B, and NK) with thymic aplasia and profound susceptibility to all infectious organisms points to SCID. X-linked SCID (most common form, ~50%) results from mutations in IL2RG encoding the gamma-c common cytokine receptor chain shared by IL-2, IL-4, IL-7, IL-9, IL-15, and IL-21 receptors, blocking lymphoid development at early stages. Bruton's disease affects only B cells, with normal T cells. DiGeorge syndrome primarily causes T-cell deficiency with intact B cells. CVID presents in early adulthood with hypogammaglobulinemia.

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

High-yield for: NEET PGINI-CETNExTFMGEUSMLEPLABMRCP

Written and medically reviewed by the StethoPrep medical team.