A 7-year-old boy presents with recurrent pyogenic infections since infancy, with persistent lymphopenia (CD3+ T-cells <200/µL), absent thymic shadow on X-ray, and undetectable serum immunoglobulins. Maternal lymphocytes are present in circulation. Which enzyme deficiency most commonly underlies this presentation?

• A Purine nucleoside phosphorylase (PNP)
• B Adenosine deaminase (ADA) ✓
• C Myeloperoxidase
• D NADPH oxidase

Explanation

Adenosine deaminase (ADA) deficiency accounts for about 15% of SCID cases and is the most common autosomal recessive form. Accumulation of deoxyadenosine and its metabolites is toxic to both T and B lymphocytes, causing combined immunodeficiency. PNP deficiency causes predominantly T-cell deficiency with relative sparing of B cells. NADPH oxidase deficiency causes Chronic Granulomatous Disease, not SCID.

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

High-yield for: NEET PGINI-CETNExTFMGEUSMLEPLABMRCP

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