First dose effect is shown by

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