Final answer:
Arrhythmia is not an etiology of myocardial infarction but rather a potential complication of it, whereas thrombus, atheroma, and embolus are direct causes of myocardial infarction.
Step-by-step explanation:
The etiology of myocardial infarction (MI), commonly known as a heart attack, involves a blockage that stops blood flow to a part of the heart, causing damage to the heart muscle. The causes of such blockages are typically associated with atherosclerosis, which is the accumulation of plaques within the arteries. Common etiologies for an MI include the formation of a thrombus (blood clot), the presence of an atheroma (plaque within the artery), or a traveling embolus that lodges in a coronary artery obstructing blood flow.
Arrhythmia, which is an abnormal rhythm of the heart, is not a direct cause of myocardial infarction. Instead, it is a complication that may result from an MI. Therefore, the correct answer to the question, "Which IS NOT an etiology of myocardial infarction?" is A. Arrhythmia.