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
First dose effect is shown by 
Select and explain. 
A. Prazocin, B. Furosemide, C. Thyronorm, D. Ceftazidime
A: Prazocin
Answer: A: Prazocin 
Explanation: 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