Which wave is masked by the large QRS complex on an ECG?

The correct answer is that atrial repolarization is masked by the large QRS complex on an ECG. During the cardiac cycle, the P wave represents atrial depolarization, followed by the QRS complex, which represents ventricular depolarization. Because the QRS complex is significantly larger in amplitude than the P wave, it can obscure the waveforms that occur concurrently or immediately before it.

Atrial repolarization occurs shortly after atrial depolarization and is often overshadowed by the powerful electrical activity of the ventricles during the QRS complex. This is due to the large number of myocardial cells involved in ventricular depolarization, which generates stronger electrical signals that can obscure the smaller signal from atrial repolarization.

While the SA node does initiate the heart's electrical activity, its wave does not have a visible representation on the ECG. Moreover, ventricular repolarization, which is seen as the T wave, occurs after the QRS complex, thus it is not masked by it. The significant difference in size and timing among these components explains why atrial repolarization is not visually represented on the ECG tracing.