This is a real-world medical entrance exam question, please give the true answer based on the question and selection. Topic name: Autonomic nervous system, Subject name: Pharmacology
[{'content': 'The rate-limiting step for nor-epinephrine 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: Ref: KD Tripathi Pharmacology 7th edition(page no;125) Catecholamines like epinephrine, norepinephrine, dopamine are synthesized from the amino acid phenylalanine. Tyrosine hydroxylase is the rate-limiting enzyme and its inhibition by alpha-methyl-p-tyrosine results in depletion of norepinephrine Tyrosine hydroxylase helps in Conversion of tyrosine to DOPA', 'role': 'assistant'}]