Classical phenylketonuria is caused by a deficiency of: (A) Phenylalanine transaminase, (B) Phenylalanine hydroxylase, (C) Tyrosine transaminase, (D) Tyrosine hydroxylase
Answer is B. 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.