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
The rate limiting step for norepinephrine synthesis is: 
Select and explain. 
A. Conversion of phenylalanine to tyrosine, B. Conversion of tyrosine to DOPA, C. Conversion of DOPA to dopamine, D. Conversion of dopamine to norepinephrine
B: Conversion of tyrosine to DOPA
Answer: B: Conversion of tyrosine to DOPA 
Explanation: 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.