A newborn is screened and found to have elevated phenylalanine (18 mg/dL) on day 3 heel-prick test. Urine pterins are normal and dihydrobiopterin reductase (DHPR) activity is normal. The diagnosis is classic phenylketonuria (PKU). Which dietary amino acid must be SUPPLEMENTED in the phenylalanine-restricted diet to prevent deficiency?
- A Methionine, as it is the first essential amino acid depleted in a phenyl-restricted diet
- B Tyrosine, because the blocked phenylalanine hydroxylase reaction prevents endogenous tyrosine synthesis ✓
- C Tryptophan, as it competes with phenylalanine for the same large neutral amino acid transporter
- D Leucine, because it becomes rate-limiting when phenylalanine intake is restricted
Explanation
In classic PKU, the enzyme phenylalanine hydroxylase (PAH) is deficient, blocking conversion of phenylalanine to tyrosine. Since this reaction is the body's major endogenous source of tyrosine, dietary tyrosine supplementation is essential in PKU management. Tyrosine becomes a conditionally essential amino acid in PKU. Without supplementation, tyrosine deficiency leads to reduced synthesis of catecholamines, melanin, and thyroid hormones. The low-phenylalanine diet must provide adequate protein through specially formulated phenylalanine-free amino acid supplements enriched with tyrosine to support normal growth and neurodevelopment.
Reference: Ghai Essential Pediatrics, 10th ed.
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Written and medically reviewed by the StethoPrep medical team.