Tetrahydrobiopterin-responsive phenylketonuria

A form of phenylketonuria characterized by mild to moderate symptoms of phenylketonuria including impaired cognitive function, seizures, and behavioral and developmental disorders, and a marked reduction and normalization of elevated phenylalanine concentrations after oral loading with tetrahydrobiopterin (BH4; sapropterin dihydrochloride), an essential cofactor of phenylalanine hydroxylase.