**Core Concept**
The QRS complex is a component of the electrocardiogram (ECG) that represents the depolarization of the ventricles in the heart. This process is crucial for the initiation of the cardiac contraction.

**Why the Correct Answer is Right**
The QRS complex is primarily due to the rapid depolarization of the ventricular muscle cells. This depolarization is initiated by the activation of the ventricular conduction system, which is triggered by the depolarization of the atrioventricular (AV) node. The electrical impulse then spreads through the bundle of His, the bundle branches, and the Purkinje fibers to the ventricular muscle cells. As a result, the ventricular muscle cells rapidly depolarize, generating the QRS complex on the ECG. This process is mediated by the opening of voltage-gated sodium channels, which allow an influx of sodium ions into the cell, resulting in rapid depolarization.

**Why Each Wrong Option is Incorrect**
**Option A:** Ventricular repolarization is the process by which the ventricular muscle cells return to their resting state, and it is represented by the T wave on the ECG, not the QRS complex.
**Option B:** Atrial depolarization is represented by the P wave on the ECG and is not related to the QRS complex.
**Option C:** Conduction through the AV node is a critical step in the cardiac conduction system, but it occurs before the QRS complex and is not responsible for generating it.

**Clinical Pearl / High-Yield Fact**
The QRS complex is a critical component of the ECG, and its duration can be used to diagnose conditions such as bundle branch blocks and ventricular hypertrophy.

**✓ Correct Answer: D. Ventricular depolarization**