**Core Concept**
Depolarization of cardiac ventricular muscle is a crucial step in the cardiac action potential, leading to muscle contraction and cardiac output. The sinoatrial (SA) node acts as the natural pacemaker, but the actual depolarization wave starts from the specialized conduction system, specifically the atrioventricular (AV) node, and then spreads to the ventricles.

**Why the Correct Answer is Right**
The correct answer is the AV node, which is a critical part of the cardiac conduction system. The AV node is located between the atria and ventricles and receives electrical signals from the atria. These signals are delayed at the AV node, allowing the atria to fully contract before the ventricles contract. The delayed depolarization at the AV node creates a refractory period, ensuring that the ventricles contract in a coordinated manner. The electrical signal from the AV node then spreads to the ventricular muscle through the bundle of His and the Purkinje fibers.

**Why Each Wrong Option is Incorrect**
**Option A:** The sinoatrial (SA) node is the natural pacemaker of the heart but does not directly initiate the depolarization of ventricular muscle.
**Option B:** The atrial muscle itself does not directly initiate the depolarization of ventricular muscle, although it does send signals to the AV node.
**Option C:** The bundle of His and Purkinje fibers are part of the cardiac conduction system but do not initiate the depolarization of ventricular muscle.

**Clinical Pearl / High-Yield Fact**
The AV node's delay in transmitting electrical signals is crucial for coordinated ventricular contraction, ensuring efficient cardiac output.

**Correct Answer: C. The AV node.**