USMLE Step 1 - Question 18

A 76 year old female with a history of coronary artery disease and prior inferior wall myocardial infarction presents with gradual onset of increased shortness of breath on exertion and lower extremity swelling. Her physical examination reveals a IV/VI holosystolic murmur at the 5th intercostals space at the mid-clavicular line. Which of the following is likely the cause of her symptoms?

A. Tricuspid valve regurgitation

B. Mitral valve regurgitation

C. Ventricular septal defect

D. Mitral valve stenosis

E. Left ventricular aneurysm

Mitral regurgitation occurs as a complication of an inferior wall myocardial infarction due to papillary muscle dysfunction resulting in failure of the mitral valve leaflets to coapt normally. Recall the cardiac anatomy of the mitral valve, specifically that there are two papillary muscles, the anterolateral and posteromedial. The anterolateral papillary muscle is perfused by the left anterior descending AND the left circumflex coronary arteries, thus dysfunction of the anterolateral papillary muscle is uncommon (since it would require 2 major artery occlusions). The posteromedial papillary muscle receives its sole blood supply from the right coronary artery (which also supplies the inferior wall in 80% of people). Thus a right coronary artery occlusion resulting in inferior wall myocardial infarction frequently causes mitral regurgitation due to concomitant papillary muscle infarction. Rarely, rupture of a papillary muscle can cause acute mitral regurgitation and cardiogenic shock which requires emergent surgical correction.

Tricuspid regurgitation (A) is another cause of a holosystolic murmur, however does not result from inferior infarction and does not commonly cause congestive heart failure. A ventricular septal defect (C) likewise can cause a holosystolic murmur and can also be a complication of inferior wall myocardial infarction, but presents more acutely a few days after the MI. Mitral valve stenosis (D) causes a diastolic murmur and is most commonly due to rheumatic heart disease. Left ventricular aneurysm (E) can cause heart failure, however does not cause a murmur and results more commonly from anterior myocardial infarction.