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
[{'content': 'Enzyme defect in Classic Phenylketonuria \nSelect and explain. \nA. Phenylalanine hydroxylase, B. Dihydrobiopterin reductase, C. Fumarylacetoacetate hydrolase, D. Homogentisate oxidase', 'role': 'user'}, {'content': 'Answer: A \nExplanation: 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', 'role': 'assistant'}]