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
Which of the following drug may cause hypertensive crisis in a patient on MAO inhibitor therapy ? 
Select and explain. 
A. Tyramine, B. Guanethidine, C. Phenobarbitone, D. Nor–epinephrine
A: Tyramine
Answer: A: Tyramine 
Explanation: None