This is a real-world medical entrance exam question, please give the true answer based on the question and selection. Topic name: None, Subject name: Biochemistry
Classical phenylketonuria is caused by a deficiency of 
Select and explain. 
A. Phenylalanine transaminase, B. Phenylalanine hydroxylase, C. Tyrosine transaminase, D. Tyrosine hydroxylase
B: Phenylalanine hydroxylase
Answer: B: Phenylalanine hydroxylase 
Explanation: 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.