Final answer:
Premature opening of the mitral valve (B) is a possible M-mode finding for constrictive pericarditis.
Step-by-step explanation:
The possible M-mode finding for constrictive pericarditis is premature opening of the mitral valve (B). The mitral valve, also known as the left atrioventricular valve, is located between the left atrium and the left ventricle. In cases of constrictive pericarditis, the stiffened or thickened pericardium restricts the movement of the heart, leading to premature opening of the mitral valve during the cardiac cycle.