Disease, maple syrup urine: Hereditary disease due to deficiency of an enzyme involved in amino acid metabolism, characterized by urine that smells like maple syrup. The three branched-chain amino acids: (leucine, isoleucine, and valine) cannot be metabolized (processed) and build up in the blood to the detriment of brain function. Untreated maple syrup urine disease (MSUD) causes mental retardation, physical disability and death.

MSUD is a component of many newborn screening programs. Treatment involves a special diet and monitoring of protein intake. The special diet lacks the three branched-chain amino acids: which are then added back to provide the protein needed for normal growth and development. Close medical care is critical because even mild illnesses can become life-threatening and require hospitalization.