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Which enzyme is deficient in classic phenylketonuria (PKU)? | QuestionStock

Representative image for question: Which enzyme is deficient in classic phenylketonuria (PKU)?

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Medicine

Which enzyme is deficient in classic phenylketonuria (PKU)?

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Which enzyme is deficient in classic phenylketonuria (PKU)?

Options:

Phenylalanine hydroxylase (PAH)
Tyrosinase
Homogentisate oxidase
Branched‑chain α‑keto acid dehydrogenase

Correct answer: Phenylalanine hydroxylase (PAH)

Explanation: Classic PKU results from deficiency of phenylalanine hydroxylase (PAH), causing elevated phenylalanine and decreased tyrosine; newborn screening programs beginning in the 1960s detect PAH deficiency to prevent intellectual disability if treated early.

Created 2026-03-10T17:31:51.073Z. Updated 2026-03-10T17:31:51.073Z.