Final answer:
Acute aortic regurgitation is usually caused by a prolapsed valve, where the blood flows backward from its normal path. This condition can be detected by listening to a heart murmur with a stethoscope.
Step-by-step explanation:
Acute aortic regurgitation is usually caused by a **prolapsed valve**. When one of the cusps of the valve is forced backward by the force of the blood, the valve fails to close properly and the blood flows backward from its normal path, resulting in regurgitation. This disruption to the normal flow of blood produces a heart murmur. Using a stethoscope, a healthcare professional can detect this abnormality.