After MI, rupture of which structure can cause acute mitral regurgitation presenting with new holosystolic murmur at the apex radiating to the axilla?

• A. Interventricular septum
• B. Papillary muscle
• C. Mitral valve leaflets
• D. Aortic valve cusp

Answer: (B)

Rupture of the papillary muscle after a myocardial infarction disrupts the mitral valve apparatus. The papillary muscles anchor the mitral valve leaflets to the wall via chordae tendineae; when one ruptures, the corresponding leaflet loses its tethering and becomes flail, causing sudden mitral regurgitation. This produces an acute, severe MR that presents as a new holosystolic murmur best heard at the apex and radiating to the axilla. The posteromedial papillary muscle is especially vulnerable after MI due to its single blood supply, which explains why papillary muscle rupture is a common cause of postinfarction acute MR. In contrast, a ventricular septal rupture would yield a murmur at the left sternal border, and an aortic valve cusp rupture would cause a diastolic murmur from aortic regurgitation, not this MR pattern.