What is the defining characteristic of mitral valve prolapse (MVP)?

Mitral valve prolapse (MVP) is primarily characterized by the abnormal displacement of one or both of the mitral valve leaflets into the left atrium during systole. This dislocation can occur due to structural changes in the valve material, which can lead to a variety of symptoms and complications.

The defining characteristic of MVP is, therefore, the fact that the valve leaflets 'prolapse' or bow back into the atrial cavity when the heart contracts. This can sometimes result in mitral regurgitation, where blood leaks back into the left atrium because the valve does not close properly. Identifying this feature is critical for diagnosis, management, and understanding the potential impact of MVP on cardiac function.

The other options presented do not accurately describe MVP. Thickening of the mitral valve leaflets is more aligned with conditions such as rheumatic heart disease or degenerative valve disease. Severe stenosis of the mitral valve pertains to a narrowing of the valve opening, which is a different pathological process altogether. Complete failure of the aortic valve involves a different set of clinical implications and is unrelated to mitral valve prolapse.