Maple syrup urine disease is a disorder of branch chain amino acids metabolism caused by a deficiency of branched-chain α-keto acid dehydrogenase complex.

MSUD is presented with five clinical phenotypes on the basis of the age at onset, the severity of symptoms and response to thiamine supplementation.

The classic MSUD is the most severe form of the disease characterized by no or very low residual enzyme activity and clinically manifested by developmental and neurological delays, encephalopathy, feeding problems, and a characteristic maple syrup odor in urine.