What ECG change is associated with ischemic injury lasting more than 20 minutes?

The correct answer is associated with ST segment elevation, which indicates a significant change in the heart's electrical activity due to ischemic injury that has persisted for more than 20 minutes. This elevation reflects ongoing cellular injury and necrosis, which can occur during an acute myocardial infarction when blood flow is severely compromised, usually due to a blocked coronary artery.

During the early phase of ischemia, the ST segment can show elevation as myocardial cells begin to depolarize and fail to repolarize properly due to the loss of the normal cellular environment needed for electrical activity. As ischemia progresses, this change can evolve into the development of pathologic Q waves. However, these Q waves do not appear immediately; they typically indicate previous myocardial damage rather than present injury.

The other choices reflect changes in less severe or different stages of ischemic conditions. T wave depression generally indicates subendocardial ischemia, often linked with angina or significant stress rather than prolonged ischemic injury. Normal sinus rhythm indicates no abnormalities in electrical activity and would not be expected in the presence of significant ischemic injury. Thus, ST segment elevation is the hallmark ECG change seen with ischemic injury lasting beyond 20 minutes, marking a critical point for medical intervention.