A 6-year-old boy has recurrent Pneumocystis jirovecii pneumonia, oral candidiasis, and failure to thrive since infancy. Flow cytometry shows absent CD3+ T cells but normal B cell counts with very low immunoglobulins. The most likely molecular defect is:

• A Bruton's tyrosine kinase (BTK) mutation
• B Adenosine deaminase (ADA) deficiency
• C IL-2 receptor gamma chain (γc) mutation ✓
• D MHC class II deficiency

Explanation

X-linked SCID caused by IL-2Rγ (common gamma chain) mutation results in absent T and NK cells but near-normal B cells that are non-functional due to absent T-cell help — this is the T−B+NK− pattern. Absent T cells lead to absent antibody production despite present B cells. ADA deficiency causes T−B−NK− SCID. BTK mutation causes X-linked agammaglobulinemia (absent B cells). MHC class II deficiency (bare lymphocyte syndrome type II) presents with low CD4 T cells and retained CD8 T cells.

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

High-yield for: NEET PGINI-CETNExTFMGEUSMLEPLABMRCP

Written and medically reviewed by the StethoPrep medical team.