This is a real-world medical entrance exam question, please give the true answer based on the question and selection. Topic name: A.N.S., Subject name: Pharmacology
[{'content': 'Vagal stimulation causes bradycardia, which can be blocked by \nSelect and explain. \nA. atenolol, B. atropine, C. doxazosin, D. phenylephrine', 'role': 'user'}, {'content': 'Answer: B \nExplanation: Bradycardia due to vagal stimulation is elicited by activation of muscarinic receptor in the heart. Atropine, which is an antagonist at M receptors, blocks bradycardia elicited by stimulation of the vagus, including reflex bradycardia due to increases in mean BP caused by vasoconstrictors.', 'role': 'assistant'}]