What is the significance of the "Q" wave in the QRS complex?

The "Q" wave in the QRS complex is significant because it represents the first downward deflection that occurs during ventricular depolarization. This is the electrical activity that triggers the ventricles to contract and pump blood to the lungs and the rest of the body. Understanding this aspect is vital for interpreting an ECG, as the presence and characteristics of the Q wave can provide important information regarding the health of the heart. For instance, in the context of myocardial infarction, the development of pathological Q waves can indicate necrosis or significant damage to the heart muscle.

The other options do not accurately describe the role of the Q wave. The Q wave does not mark the end of atrial contraction, does not signify ventricular repolarization, and does not indicate a normal heart rhythm on its own; instead, these aspects pertain to different phases of the cardiac cycle and can be assessed through various elements of the ECG. Thus, acknowledging the Q wave as a vital part of ventricular depolarization reinforces its clinical relevance in cardiology.