A 58-year-old man with acute anterior wall myocardial infarction is admitted to the ICU. On day 5 post-MI, he suddenly develops acute dyspnea, hypotension, and a new harsh holosystolic murmur best heard at the apex. Echocardiography shows acute severe mitral regurgitation with normal left ventricular systolic function. The pathological process marked **C** in the healing timeline diagram is responsible for the structural weakness that predisposes to this complication. Which of the following best explains why the posteromedial papillary muscle is more vulnerable to rupture than the anterolateral papillary muscle in this clinical scenario?

Explanation

## Why option 1 (PDA supply to posteromedial papillary muscle) is correct Days 4–7 post-MI (the period marked **C** in the diagram) represent the phase of maximum risk for mechanical complications, including papillary muscle rupture. During this interval, macrophages have removed necrotic debris but granulation tissue and collagen have not yet formed, leaving the myocardium structurally weakest. The posteromedial papillary muscle is supplied by a single coronary artery—the posterior descending artery (PDA)—which makes it uniquely vulnerable to ischemic necrosis when the RCA is occluded. In contrast, the anterolateral papillary muscle receives dual blood supply from both the LAD and LCx, providing collateral protection. When the posteromedial papillary muscle undergoes necrosis and is weakened by macrophage-mediated debris removal (stage **C**), it ruptures, causing acute mitral regurgitation, pulmonary edema, and hemodynamic collapse. This is the most common papillary muscle rupture pattern in acute MI (Robbins 10e Ch 12; Harrison 21e Ch 269). ## Why each distractor is wrong - **Option 0 (dual supply to posteromedial)**: This reverses the anatomical fact. The posteromedial papillary muscle has *single* supply from the PDA, not dual supply. Dual supply is the feature of the anterolateral papillary muscle, which protects it from rupture. - **Option 2 (anterolateral supplied by PDA)**: Anatomically incorrect. The anterolateral papillary muscle is supplied by the LAD and LCx, not the PDA. This confusion would lead to the wrong prediction about which muscle ruptures. - **Option 3 (structural size and mechanical stress)**: While papillary muscle rupture does occur in the context of mechanical stress, the *selectivity* for posteromedial rupture is determined by vascular anatomy (single PDA supply), not muscle size. Size and stress do not explain the epidemiological pattern. **High-Yield:** Posteromedial papillary muscle rupture >> anterolateral because of single PDA supply; occurs at days 4–7 when macrophages weaken the wall before collagen deposition. [cite: Robbins 10e Ch 12; Harrison 21e Ch 269]