A 68-year-old patient with severe aortic stenosis undergoes exercise stress testing. During peak exercise, cardiac output fails to increase appropriately. Which physiological mechanism BEST explains the blunted cardiac output response in fixed obstructive valvular disease?

• A Reduced chronotropic response due to increased vagal tone
• B Impaired Starling mechanism due to elevated left ventricular end-diastolic pressure
• C Decreased coronary perfusion pressure leading to subendocardial ischemia reducing contractility
• D Inability to reduce afterload sufficiently against the fixed outflow obstruction ✓

Explanation

In severe aortic stenosis, the fixed obstruction limits the stroke volume increase required during exercise, as the LV cannot further reduce its effective afterload beyond the stenotic orifice. While subendocardial ischemia does occur, the primary mechanism limiting cardiac output augmentation is the inability to reduce effective afterload (the stenotic valve acts as a fixed resistance). Options A and C may contribute but are secondary phenomena; D is a consequence, not the primary hemodynamic constraint.

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

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