Phenylketonuria (PKU) is caused by deficiency of which enzyme, and what is the typical chronic dietary management?

• A. Deficiency of Hepatic Phenylalanine Hydroxylase With High Phenylalanine and Low Tyrosine; Phenylalanine-Free Diet With Tyrosine Supplementation
• B. Deficiency of Hepatic Tyrosine Hydroxylase With High Tyrosine and Low Phenylalanine; Tyrosine-Rich Diet
• C. Excess Phenylalanine Intake; No Dietary Restriction
• D. Deficiency of Phenylalanine; Treat With Phenylalanine Supplementation

Answer: (A)

Phenylketonuria is caused by a deficiency of hepatic phenylalanine hydroxylase, the enzyme that normally converts phenylalanine to tyrosine. When this step can’t occur, phenylalanine builds up in the blood and tissues while tyrosine becomes deficient, which disrupts melanin production and other tyrosine-derived pathways. The long-term dietary approach is to restrict phenylalanine intake to keep levels normal and provide tyrosine through supplementation because tyrosine becomes essential in this condition. This combination—high phenylalanine, low tyrosine—and the treatment with a phenylalanine-restricted diet plus tyrosine supplementation best fits the standard management of PKU.