This is a real-world medical entrance exam question, please give the true answer based on the question and selection. Topic name: Proteins and Amino Acids, Subject name: Biochemistry
[{'content': 'PKU is due to deficiency of which enzyme: \nSelect and explain. \nA. Phenylalanine reductase, B. Phenylalanine hydroxylase, C. Tyrosine hydroxylase, D. Tyrosine reductase', 'role': 'user'}, {'content': 'Answer: B \nExplanation: Ans. (b) Phenylalanine hydroxylaseRef: Harrisons, 19th ed. pg. 434e1* PKU is an autosomal recessive disorder characterized by an increased concentration of phenylalanine and its byproducts in body fluids and by severe mental retardation if untreated in infancy.* It results from reduced activity of phenylalanine hydroxylase.* The accumulation of phenylalanine:# Inhibits the transport of other amino acids required for protein or neurotransmitter synthesis# Reduces synthesis and increases degradation of myelin, and leads to inadequate formation of norepinephrine and serotonin.* To prevent intellectual disability, diagnosis and initiation of dietary therapy must be started before 2 weeks of age', 'role': 'assistant'}]