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A 7-month-old girl was picked up on newborn screening with raised phenylalanine and started on a phenylalanine-restricted diet. Her phenylalanine levels have since stayed in the target range, yet she has developed truncal hypotonia, limb rigidity, drooling, temperature instability and seizures. Serum prolactin is raised, and CSF shows low homovanillic acid and 5-hydroxyindoleacetic acid. Which defect best explains her course?
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