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: Pharmacology
[{'content': 'Which of the following drug may cause hypertensive crisis in a patient on MAO inhibitor therapy ? \nSelect and explain. \nA. Tyramine, B. Guanethidine, C. Phenobarbitone, D. Nor–epinephrine', 'role': 'user'}, {'content': 'Answer: A \nExplanation: None', 'role': 'assistant'}]