Which finding is most consistent with this M-mode image?

Comprehensive and Detailed Explanation From Exact Extract:

This M-mode echocardiographic image shows thickened mitral valve leaflets with a characteristic "doming" or "hockey-stick" appearance during diastole, which is classic for rheumatic mitral stenosis. Rheumatic mitral stenosis leads to leaflet thickening, restricted opening, and calcification, which alters the normal mitral valve motion on M-mode.

Mitral valve prolapse would show systolic displacement of the leaflets into the left atrium, typically later in systole, not doming in diastole. Mitral valve annuloplasty ring would appear as a bright echogenic line around the annulus but is not seen in this image. Systolic anterior motion (SAM) of the mitral valve is usually seen in hypertrophic cardiomyopathy and presents as anterior motion during systole, not the diastolic pattern shown.

This classical M-mode appearance is described in "Textbook of Clinical Echocardiography, 6e", Chapter on Rheumatic Valve Disease【20:385-390†Textbook of Clinical Echocardiography】.