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X-linked SCID immunophenotype — MRCPCH TAS MCQ

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HardImmunologyX-linked SCID immunophenotypeMRCPCH TAS

A 3-month-old boy has chronic diarrhoea, failure to thrive and recurrent viral infections. Lymphocyte subsets show very low T cells, absent NK cells and preserved B-cell numbers. What is the most likely defect?

Educational content. Not a substitute for clinical judgement or local policy.

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Correct answer: B — Common gamma-chain cytokine receptor defect

X-linked SCID due to common gamma-chain defects typically causes T-cell and NK-cell deficiency with B cells present but functionally ineffective. BTK deficiency causes X-linked agammaglobulinaemia with absent mature B cells after maternal IgG wanes. The pearl is that live vaccines can be dangerous in severe T-cell immunodeficiency.

Reference: Nelson Textbook of Pediatrics; Green Book immunisation guidance; RCPCH Theory examination syllabi