What does the QRS complex on an ECG signify?

The QRS complex on an electrocardiogram (ECG) signifies ventricular contraction, also known as ventricular depolarization. This component of the ECG represents the electrical activity that triggers the ventricles of the heart to contract and pump blood to the lungs and the rest of the body. The QRS complex appears as a series of spikes and dips on the ECG tracing, where the sharp spikes indicate the rapid depolarization of the ventricles as the electrical impulse moves through the cardiac conduction system.

Understanding the significance of the QRS complex is crucial for interpreting the heart's rhythm and identifying potential cardiac issues. The other choices relate to different phases of cardiac activity; for instance, atrial contraction corresponds to the P wave, the resting potential refers to the baseline state of the cardiac cells when not depolarizing, and ventricular repolarization is represented by the T wave. Thus, the QRS complex is specifically associated with the contraction of the ventricles, making that the correct answer.