Hexamethonium, a ganglion-blocking drug, produces profound fall in blood pressure on standing but little change in supine blood pressure. The reason for this pattern is:

• A Standing requires reflex sympathetic vasoconstriction, which ganglionic blockade abolishes ✓
• B Direct relaxation of capacitance vessels masks the supine effect
• C Supine blood pressure is maintained entirely by circulating epinephrine from the adrenal medulla
• D Ganglionic blockade increases venous return in the upright position

Explanation

On standing, baroreceptors trigger reflex sympathetic vasoconstriction and tachycardia, both of which require nicotinic transmission through paravertebral ganglia. Hexamethonium blocks Nn receptors at all autonomic ganglia, abolishing this reflex and causing marked postural hypotension. In the supine position little reflex tone is needed, so pressure is relatively preserved. Option C is incorrect because adrenal output contributes far less than neural tone.

Reference: Katzung's Basic and Clinical Pharmacology, 15th ed.

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