Which mechanism is most commonly implicated in myocardial infarction?

• A. Vasospasm during occlusion in atheroma
• B. Thrombus build-up obstructs the artery
• C. Embolus travels to the coronary artery
• D. Bacterial infection of the artery

Answer: (B)

The key idea is that myocardial infarction most often happens when a thrombus forms over a ruptured atherosclerotic plaque in a coronary artery, acutely blocking blood flow to the heart muscle. Atherosclerotic plaques develop a lipid-rich core and a fragile fibrous cap; when the cap ruptures, subendothelial collagen and thrombogenic material are exposed, triggering platelet adhesion and aggregation plus activation of the coagulation cascade. This rapid thrombus formation can partially or completely occlude the artery, causing ischemia and, if perfusion isn’t restored, infarction of the downstream myocardium.

Other mechanisms can cause ischemia, but they’re less common as the primary cause of MI. Vasospasm can provoke chest pain and, in some situations, contribute to ischemia, but the predominant driver of infarction is thrombotic occlusion at a ruptured plaque. An embolus to the coronary artery is possible but is not the typical scenario. Bacterial infection of the artery is not a common mechanism for acute MI.

So, the best explanation is a thrombus obstructing a coronary artery after plaque rupture, leading to myocardial infarction.