Final answer:
Ischemic heart disease commonly causes S-T segment depression on an ECG, indicating myocardial ischemia due to insufficient oxygen delivery to the heart muscle.
Step-by-step explanation:
Ischemic heart disease often causes S-T segment depression on the electrocardiogram (ECG). This specific ECG change is indicative of myocardial ischemia, which occurs when there is a reduced blood flow to the heart's muscle tissue, leading to insufficient oxygen delivery to the myocardium. S-T segment depression is different from elevated or inverted P waves, which can indicate other cardiac conditions, and it is distinct from the absence of a QRS complex, which might suggest a more severe blockage of cardiac electrical activity.
In the case of ischemic heart disease, the S-T segment can appear depressed below the baseline of the ECG. The ST segment represents the period between the end of the ventricular depolarization and the beginning of the repolarization of the ventricles. An elevated ST segment is often seen in acute myocardial infarctions (MI), whereas ST depression can signal hypoxia or ischemia. Therefore, the correct answer to the student's question is 'C. S-T depression.'