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
The rate-limiting enzyme in the synthesis of dopamine is which of the following? 
Select and explain. 
A. Tyrosine hydroxylase, B. Dopa decarboxylase, C. Homovanillic acid, D. Dopamine beta-hydroxylase
A: Tyrosine hydroxylase
Answer: A: Tyrosine hydroxylase 
Explanation: 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.