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': 'The rate-limiting enzyme in the synthesis of dopamine is which of the following? \nSelect and explain. \nA. Tyrosine hydroxylase, B. Dopa decarboxylase, C. Homovanillic acid, D. Dopamine beta-hydroxylase', 'role': 'user'}, {'content': 'Answer: A \nExplanation: The first step in the chemical synthesis of dopamine, epinephrine, and norepinephrine is the conversion of L-tyrosine to L-hydroxyphenylalanine, which is catalyzed by tyrosine hydroxylase. This is the rate-limiting step.', 'role': 'assistant'}]