**Core Concept**
The QRS complex is a critical component of the electrocardiogram (ECG) that represents the depolarization of the ventricles. This complex is a result of the rapid conduction of electrical impulses from the atrioventricular (AV) node to the ventricular muscle cells.

**Why the Correct Answer is Right**
The QRS complex is caused by the depolarization of the ventricular muscle cells, specifically the rapid upstroke of the action potential in the ventricular myocytes. This is mediated by the opening of voltage-gated sodium channels, which allows a rapid influx of sodium ions into the cell. The rapid depolarization of the ventricular muscle cells results in the characteristic slurred upstroke of the QRS complex on the ECG. The QRS complex is also influenced by the conduction velocity of the electrical impulses through the ventricular muscle cells.

**Why Each Wrong Option is Incorrect**
**Option A:** This option is incorrect because the P wave represents the depolarization of the atria, not the ventricles.
**Option B:** This option is incorrect because the T wave represents the repolarization of the ventricles, not the depolarization.
**Option C:** This option is incorrect because the PR interval represents the time from the onset of the P wave to the start of the QRS complex, which is the period of electrical conduction from the atria to the ventricles.

**Clinical Pearl / High-Yield Fact**
The QRS complex is a critical component of the ECG that can be used to diagnose various cardiac conditions, including ventricular hypertrophy, bundle branch blocks, and ventricular tachycardia. It is essential for clinicians to accurately identify and interpret the QRS complex to make informed decisions about patient care.

**Correct Answer: C. Depolarization of the ventricular muscle cells.**