Prazosin differs from phentolamine in that prazosin:
- A Blocks both alpha-1 and alpha-2 adrenoceptors
- B Is indicated for pheochromocytoma pre-operatively
- C Causes reflex tachycardia less frequently due to selective alpha-1 blockade ✓
- D Has a shorter duration of action
Correct answer: C. Causes reflex tachycardia less frequently due to selective alpha-1 blockade
Explanation
Prazosin selectively blocks alpha-1 receptors while sparing presynaptic alpha-2 receptors; this preserves the negative feedback on noradrenaline release, so reflex tachycardia is minimal. Phentolamine blocks both alpha-1 and alpha-2, abolishing presynaptic feedback and causing significant reflex tachycardia. Phentolamine (not prazosin) is preferred for pheochromocytoma because of its faster, titratable alpha-2 blockade and rapid onset.
Reference: KD Tripathi, Essentials of Medical Pharmacology, 8th ed.
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