Which MI territory is usually associated with new-onset mitral regurgitation?

• A. Septal
• B. Lateral
• C. Inferior
• D. Anterior

Answer: (C)

New-onset mitral regurgitation after a myocardial infarction is most often due to rupture of a papillary muscle that supports the mitral valve. The posteromedial papillary muscle, which anchors the posterior leaflet, has a single blood supply from the posterior descending artery. An inferior MI, typically arising from the right coronary artery and involving the PDA territory, puts this muscle at risk of ischemic rupture. When it ruptures, the valve leaflets become tethered abnormally (a flail leaflet), causing sudden, severe MR and acute heart failure a few days after the infarct.

In contrast, the anterior or lateral infarctions involve the anterolateral papillary muscle, which has dual blood supply from the left anterior descending and left circumflex arteries, making rupture less likely. So the inferior territory is the one usually associated with new-onset MR after MI.