<prompt> PKU is a congenital amino acid metabolic disorder. In one of the following rare variants of PKU Dihydro Biopterin synthesis is affected. The enzyme deficient isHistidine decarboxylasePhenylalanine hydroxylaseDihydropterin reductaseTyrosine deficiency  <response>: Atypical form of phenylketonuria is due to deficiency of dihydrobiopterin reductase which is required for converion of H2-biopterin intorin. H4-biopterin is required for oxidation of phenylalanine because it is a cofactor for phenylalanine hydroxylase. 