This is a real-world medical entrance exam question, please give the true answer based on the question and selection. Topic name: Metabolism of protein and amino acid, Subject name: Biochemistry
Phenylketonuria is caused by deficiency of 
Select and explain. 
A. Tyrosine transminase, B. Tyrosine hydroxylase, C. Phenylalanine transaminase, D. Phenylalanine hydroxylase
D: Phenylalanine hydroxylase
Answer: D: Phenylalanine hydroxylase 
Explanation: Deficiency of Phenylalanine hydroxylase is the cause of PKU. The genetic mutation may be in such a way that either the enzyme is not synthesized or a non-functional enzyme is synthesized.Ref: DM Vasudevan - Textbook of Biochemistry, 7th edition, page no: 236