Which of the following aminoacidopathies is NOT caused by an enzyme deficiency?

Cystinuria is indeed not caused by an enzyme deficiency but rather by a defect in the transport system responsible for reabsorbing certain amino acids in the kidneys. This condition leads to excessive excretion of cystine and other amino acids in the urine, which can cause kidney stones.

In contrast, the other conditions listed result from specific enzyme deficiencies. For instance, phenylketonuria is due to a deficiency in the enzyme phenylalanine hydroxylase, which is crucial for metabolizing phenylalanine. Alkaptonuria arises from a lack of the enzyme homogentisate oxidase, leading to the accumulation of homogentisic acid in the body. Maple syrup disease is caused by a deficiency in the branched-chain alpha-keto acid dehydrogenase complex, which is vital for the metabolism of certain amino acids.

The distinction lies in the underlying mechanisms of these conditions; while the other disorders stem from enzymatic failures, cystinuria stems from a transport mechanism issue rather than enzyme function.