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Phenylketonuria enzymology — MRCPCH TAS MCQ

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ModerateGeneticsPhenylketonuria enzymologyMRCPCH TAS

A baby identified on newborn screening has high phenylalanine and normal examination. The parents are unaffected and unrelated. What is the most likely biochemical defect?

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Correct answer: C — Phenylalanine hydroxylase deficiency

Classical phenylketonuria is caused by phenylalanine hydroxylase deficiency, leading to raised phenylalanine. Branched-chain alpha-ketoacid dehydrogenase deficiency causes maple syrup urine disease with leucine, isoleucine and valine accumulation. Early detection matters because dietary phenylalanine restriction prevents neurodevelopmental injury.

Reference: Nelson Textbook of Pediatrics; UK newborn screening resources; RCPCH Theory examination syllabi