Classical phenylketonuria is caused by a deficiency of

Classical phenylketonuria (hyperphenylalaninemia type I) → Deficiency of phenylalanine hydroxylase. Atypical phenylketonuria (hyperphenylalaninemia type II and III) → Defect in dihydrobiopterin reductase. Hyperphenylalaninemia type IV and V → Defect in dihydrobiopterin synthesis.