A. An increased capillary refill time suggests reduced perfusion and may indicate decreased cardiac output, which is critical to assess in patients with atrial fibrillation as it can lead to hemodynamic instability. B. A rumbling heart murmur may suggest valvular disease but is not specifically indicative of decreased cardiac output in this scenario. C. Intermittent claudication typically indicates peripheral arterial disease and is not a direct sign of decreased cardiac output. D. Jugular venous distension can indicate fluid overload or right-sided heart failure, but it is not the most direct indicator of decreased cardiac output compared to capillary refill time.