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
Enzyme defect in Classic Phenylketonuria 
Select and explain. 
A. Phenylalanine hydroxylase, B. Dihydrobiopterin reductase, C. Fumarylacetoacetate hydrolase, D. Homogentisate oxidase
A: Phenylalanine hydroxylase
Answer: A: Phenylalanine hydroxylase 
Explanation: Hyperphenylalaninemias arise from defects in phenylalanine hydroxylase (type I, classic phenylketonuria (PKU), frequency 1 in 10,000 bihs), in dihydrobiopterin reductase (types II and III), or in dihydrobiopterin biosynthesis (types IV and V).Ref: Harper&;s Biochemistry; 30th edition; Chapter 29 Catabolism of the Carbon Skeletons of Amino Acids