This is a real-world medical entrance exam question, please give the true answer based on the question and selection. Topic name: Adrenergic System, Subject name: Pharmacology
[{'content': 'The rate limiting step for norepinephrine synthesis is: \nSelect and explain. \nA. Conversion of phenylalanine to tyrosine, B. Conversion of tyrosine to DOPA, C. Conversion of DOPA to dopamine, D. Conversion of dopamine to norepinephrine', 'role': 'user'}, {'content': 'Answer: B \nExplanation: Ans. (B) Conversion of tyrosine to DOPA(Ref: Katzung 10/e p78; KDT 8th/e p136)Rate limiting enzyme in norepinephrine synthesis is tyrosine hydroxylase that catalyses the conversion of tyrosine to dopa. In ACh synthesis, rate limiting step is uptake of choline by the neurons.', 'role': 'assistant'}]