Final answer:
The most common valvular lesion in horses is mitral regurgitation, where the mitral valve fails to close properly, resulting in backflow of blood from the left ventricle to the left atrium.
Step-by-step explanation:
The most common valvular lesion in horses is mitral regurgitation. It is detected in approximately 2 percent of the horse population. Mitral regurgitation occurs when the mitral valve fails to close properly, leading to the backflow of blood from the left ventricle to the left atrium. Stenosis, which is the narrowing or rigidity of the heart valves, is not the most common valvular lesion in horses.