A 55-year-old man with severe aortic stenosis develops syncope on exertion. The primary hemodynamic mechanism responsible for this symptom is:

• A Decreased preload due to impaired venous return during exertion
• B Arrhythmia triggered by increased catecholamine release
• C Inability to increase cardiac output across the fixed obstruction while peripheral resistance falls ✓
• D Coronary steal phenomenon reducing myocardial perfusion pressure

Explanation

In severe aortic stenosis, the fixed valvular obstruction limits the heart's ability to augment cardiac output during exercise. During exertion, skeletal muscle vasodilation reduces systemic vascular resistance markedly, but since cardiac output cannot rise proportionally due to the obstruction, arterial blood pressure falls, causing cerebral hypoperfusion and syncope. Arrhythmias can contribute but are not the primary mechanism; coronary steal is not the dominant cause here.

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

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