In PKU dietary management, which amino acid becomes conditionally essential and is supplemented?

• A. Methionine
• B. Valine
• C. Tryptophan
• D. Tyrosine

Answer: (D)

In PKU, the enzyme that converts phenylalanine to tyrosine is deficient, so tyrosine can no longer be made from phenylalanine. Because of this block, tyrosine becomes conditionally essential—it must be supplied in the diet to meet the body's needs for protein synthesis and for tyrosine-derived compounds such as catecholamines and thyroid hormones. PKU dietary formulas are designed to limit phenylalanine while providing adequate tyrosine to prevent deficiency. The other amino acids listed are not made conditionally essential by PKU; they remain essential amino acids that need to be obtained from the diet, though their amounts are managed within the PKU diet.