In untreated classic phenylketonuria, which amino acid becomes an essential dietary requirement?

• A Glycine
• B Tryptophan
• C Methionine
• D Tyrosine ✓

Explanation

Phenylalanine hydroxylase normally converts phenylalanine to tyrosine, making tyrosine nonessential in healthy individuals. When the enzyme is deficient, tyrosine can no longer be synthesized endogenously and must be supplied in the diet, while phenylalanine must be restricted. This is why PKU diets are both low in phenylalanine and supplemented with tyrosine. Tryptophan, methionine, and glycine remain essential or nonessential independent of this block.

Reference: Lippincott's Illustrated Reviews: Biochemistry, 8th ed.

High-yield for: NEET PGINI-CETNExTFMGEUSMLEPLABMRCP

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