A patient with severe aortic stenosis develops syncope on exertion. The pathophysiological mechanism of exertional syncope in this condition is BEST explained by which of the following?

• A Fixed cardiac output cannot increase during exercise, causing cerebral hypoperfusion when peripheral vascular resistance falls ✓
• B Reflex vagal bradycardia triggered by elevated left ventricular pressure
• C Coronary steal phenomenon diverting blood from cerebral circulation
• D Paradoxical embolism through a patent foramen ovale opened by elevated right atrial pressure

Explanation

In severe aortic stenosis, the fixed obstruction limits the increase in cardiac output during exercise. During exercise, normal skeletal muscle vasodilation reduces peripheral vascular resistance; in a healthy person, cardiac output rises proportionally to maintain blood pressure. In aortic stenosis, cardiac output remains fixed while peripheral resistance drops, causing a net fall in mean arterial pressure and cerebral perfusion pressure, leading to syncope. Option B (Bezold-Jarisch reflex) may contribute but is not the primary mechanism; options C and D are not relevant mechanisms here.

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

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