Which of the following best describes mitral regurgitation?

Mitral regurgitation is defined by the retrograde flow of blood from the left ventricle back into the left atrium during systole. This condition occurs when the mitral valve does not close properly, allowing blood to flow backward instead of efficiently moving forward into the aorta. The retrograde flow is a direct consequence of this inadequate closure and can lead to volume overload in the left atrium, causing various symptoms and complications.

The other choices describe different cardiac issues or mechanisms that do not directly define mitral regurgitation. For instance, narrowing of a valve refers to stenosis, which is characterized by restricted blood flow forward through the valve rather than backward. Similarly, uneven closure of valve flaps could describe a potential cause of regurgitation, but it does not capture the essence of the problem, which is primarily about the backward flow of blood. Calcification of the leaflet edges might be associated with degenerative changes in the valve but does not specifically illustrate the primary abnormality seen in mitral regurgitation, which is the failure of the valve to prevent retrograde flow.