tetrahydrobiopterin-responsive hyperphenylalaninemia/phenylketonuria (MONDO_0017389, a Monarch Disease Ontology id) can be described as follows. Tetrahydrobiopterin-responsive hyperphenylalaninemia/ phenylketonuria (BH4-responsive hyperphenylalaninemia/ phenylketonuria) is a form of phenylketonuria (PKU), an inborn error of amino acid metabolism, characterized by mild to moderate symptoms of PKU 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. Also known as: BH4-responsive HPA/PKU, BH4-responsive hyperphenylalaninemia/phenylketonuria, tetrahydrobiopterin-responsive HPA/PKU.