For example, phenylketonuria results from a deficiency of phenylalanine hydroxylase (PAH), leading to phenylalanine accumulation; medium-chain acyl-CoA dehydrogenase (MCAD) deficiency reflects insufficient MCAD activity and yields characteristic disruptions in medium-chain fatty acid oxidation; and maple syrup urine disease is caused by a deficiency of the branched-chain α-ketoacid dehydrogenase complex (BCKDC), producing elevations of branched-chain amino acids and their related metabolites. Here, PAH is linked to phenylketonuria.