The biochemical defect in phenylketonuria (PKU) is a deficiency of the liver enzyme phenylalanine hydroxylase, which catalyzes the para-hydroxylation of phenylalanine to yield which amino acid?

• A. Threonine
• B. Alanine
• C. Methionine
• D. Tyrosine

Answer: (D)

Phenylalanine hydroxylase performs para-hydroxylation on phenylalanine to form tyrosine. In PKU, the liver enzyme is deficient, so this conversion is impaired. As a result, phenylalanine builds up while tyrosine levels drop, making tyrosine the amino acid produced by this specific metabolic step and, in normal physiology, a downstream product of phenylalanine. The other amino acids listed are not generated by this enzymatic reaction.