Definition of Maple syrup urine disease

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Maple syrup urine disease: A hereditary disease that is due to deficiency of an enzyme involved in amino acid metabolism, characterized by urine that smells like maple syrup. In maple syrup urine disease, the three branched-chain amino acids (leucine, isoleucine, and valine) cannot be metabolized (processed), and they build up in the blood, causing problems with brain function and leading to mental retardation, physical disability, and death, if not treated. Treatment involves use of a special diet and monitoring of protein intake.