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
[{'content': 'Classical phenylketonuria is caused by a deficiency of \nSelect and explain. \nA. Phenylalanine transaminase, B. Phenylalanine hydroxylase, C. Tyrosine transaminase, D. Tyrosine hydroxylase', 'role': 'user'}, {'content': 'Answer: B \nExplanation: Classical phenylketonuria (hyperphenylalaninemia type I)\xa0→\xa0Deficiency of phenylalanine hydroxylase.\nAtypical phenylketonuria (hyperphenylalaninemia type II and III) →\xa0Defect in dihydrobiopterin reductase.\nHyperphenylalaninemia type IV and V\xa0→\xa0Defect in dihydrobiopterin synthesis.', 'role': 'assistant'}]