Orthostatic hypotension in a patient with pure autonomic failure (e.g., multiple system atrophy) is due to failure of which specific reflex arc?

• A Failure of sympathetic vasoconstrictor reflex: carotid/aortic baroreceptors detect hypotension → NTS → RVLM, but preganglionic sympathetic neurons are dysfunctional and cannot increase vascular resistance ✓
• B Failure of the Bainbridge reflex: venous return from legs activates right atrial stretch receptors, but the afferent arm is denervated
• C Parasympathetic overactivity: standing activates vagal efferents producing paradoxical vasodilation
• D Hypersensitivity of baroreceptors: excessive firing at normal blood pressure prevents compensatory tachycardia

Explanation

In pure autonomic failure/multiple system atrophy, the preganglionic or postganglionic sympathetic neurons in the intermediolateral cell column degenerate. On standing, venous pooling reduces cardiac output and arterial pressure, normally triggering baroreceptor unloading → increased sympathetic outflow from the rostral ventrolateral medulla (RVLM) → vasoconstriction and tachycardia. When the sympathetic efferent arm fails, blood pressure falls without adequate compensatory vasoconstriction. The afferent arc (baroreceptors to NTS) may be intact. Bainbridge reflex (option B) is a cardiac reflex for volume-driven tachycardia. Option C is incorrect as parasympathetic activity does not directly vasodilate systemic resistance vessels. Option D describes hypersensitivity, the opposite of what occurs.

Reference: Guyton & Hall, Textbook of Medical Physiology, 14th ed.

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