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ST elevation in lead aVR, accompanied by widespread horizontal ST depression in other leads, often suggests significant ischemia, particularly indicative of left main coronary artery occlusion. This pattern arises because the left main coronary artery supplies the majority of the myocardium. When there is a substantial blockage in this artery, it can lead to widespread ischemia in the heart muscle, causing ST segment depression in many leads due to repolarization changes.

The elevation in aVR occurs due to the high degree of ischemia affecting the anterior and lateral walls of the heart. This finding, in conjunction with the ST depression seen in other leads, underscores the severity of the situation, as it is often a precursor to a large anterior myocardial infarction.

In contrast, other conditions like myocardial infarction, pulmonary embolism, and aortic dissection do not typically present with this specific combination of ECG changes, making them less likely in this scenario. This understanding of how to interpret these specific ECG findings is crucial for identifying a serious coronary condition, such as left main coronary artery occlusion, prompting immediate clinical intervention.