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': 'First dose effect is shown by \nSelect and explain. \nA. Prazocin, B. Furosemide, C. Thyronorm, D. Ceftazidime', 'role': 'user'}, {'content': 'Answer: A \nExplanation: Ans. is \'a\' i.e., Prazocin "First dose effect (postural hypotension) is now uncommon and occurs mainly with drugs having an action on veins (a-adrenoreceptor blockers like prazosin, ACE inhibitors) when baroreceptors reflex is impared e.g. old age or with contracted intravascular volume following diuretics" -- Lawrence 9th/ep. 489', 'role': 'assistant'}]