In PKU, which amino acid becomes deficient due to the metabolic block?

• A. Tyrosine
• B. Phenylalanine
• C. Methionine
• D. Lysine

Answer: (A)

In PKU, the block is the inability to convert phenylalanine to tyrosine because phenylalanine hydroxylase is deficient. This means phenylalanine builds up while tyrosine production drops, making tyrosine the amino acid that becomes deficient and, in PKU, it must be supplied from the diet (it becomes conditionally essential). So tyrosine is the correct focus. The other amino acids listed aren’t the ones deficient due to this metabolic block: phenylalanine itself accumulates, not decreases, and methionine and lysine aren’t specifically impacted by this defect.