Final answer:
The "swishing" sound in a heart murmur during the cardiac cycle is typically caused by blood flowing back through a leaky valve, known as regurgitation.
Step-by-step explanation:
A patient with a heart murmur exhibiting a "swishing" sound during the cardiac cycle is likely experiencing this sound due to blood flowing back through a leaky valve, which is option a. This condition, known as regurgitation, happens when the valve fails to close properly, disrupting the normal one-way flow of blood and causing the blood to flow backward. Normal heart sounds include S1 ("lub"), when the atrioventricular valves close, and S2 ("dub"), when the semilunar valves close. Murmurs are typically identified through auscultation using a stethoscope and can vary in severity. A condition such as valve prolapse, where one of the valve's cusps is forced backward by blood, can lead to significant regurgitation and an audible murmur.