What is myxomatous redundant mitral valve prolapse?

Birth defect: It's abnormal tissue composing the mitral valve from birth. Myxomatous tissue is thickened, exuberant, and stretches - thus doesn't provide proper support for the mitral valve which then "bows backward" under the force of cardiac contraction and "prolapses" (sticks its neck out) into the left atrium during systole (where it doesn't belong). It can lead to mitral regurgitation (leaking).