A patient with pure autonomic failure has orthostatic hypotension but a normal heart rate response on standing. Which autonomic pathway defect explains this selective clinical pattern?

• A Failure of cardiac sympathetic efferents with intact vasomotor sympathetic efferents
• B Bilateral carotid sinus hypersensitivity preventing heart rate increase
• C Impaired vagal withdrawal that normally produces the initial tachycardia on standing
• D Failure of peripheral vasomotor sympathetic efferents to arterioles with intact cardiac autonomic innervation ✓

Explanation

The normal heart rate increase on standing (phase I of orthostatic response, lasting first 15 seconds) is primarily mediated by vagal withdrawal and cardiac sympathetic activation — this is intact. The blood pressure failure indicates absent or markedly impaired peripheral vasomotor sympathetic efferents to arterioles and veins, which fail to increase TPR to compensate for gravitational venous pooling. Pure autonomic failure classically shows this pattern because peripheral sympathetic vasomotor fibers are selectively lost before cardiac sympathetic fibers. Options A, C, and D describe alternative patterns that do not match this presentation.

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

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