Which finding is most reliable in predicting severe aortic stenosis?

The presence of a delayed carotid upstroke is the most reliable finding in predicting severe aortic stenosis. This phenomenon occurs due to the reduced stroke volume and prolonged ejection time associated with the narrowing of the aortic valve, leading to a slower rise in the carotid pulse. As the heart struggles to eject blood through the constricted valve, the pulse wave travels more slowly through the arterial system, resulting in this characteristic delayed upstroke.

In contrast, a fast pulse rate may suggest increased heart rate but does not directly indicate the severity of aortic stenosis; it could be influenced by various factors including anxiety, fever, or other cardiac conditions. Low blood pressure is not specific enough to indicate severe aortic stenosis as it can occur in many other acute or chronic conditions that affect fluid status or cardiac output. The presence of a bounding pulse is more typically associated with certain types of regurgitant lesions rather than stenotic lesions, and does not align with the characteristics of aortic stenosis, which typically results in a weaker, diminished pulse due to reduced blood flow.

Thus, of all these findings, the delayed carotid upstroke is the strongest indicator of significant obstruction caused by severe aortic stenosis.