Maple syrup urine disease (MSUD) is caused by the inability to metabolize the branched-chain amino acids leucine, isoleucine, and valine. The disease is called MSUD because urine from affected people smells like maple syrup.
In the most severe form, MSUD causes severe acidosis during the first week of life. This is characterized by progressively poorer feeding, vomiting, seizures, lethargy, and finally coma. Untreated infants may die in the first few weeks of life in severe forms of the disease. MSUD also occurs in an intermittent form and a mild form. Even in the mildest form, infections can cause mental retardation and bouts of acidosis.
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