Okay, let's tackle this question. The question is about where depolarization of human ventricular muscle starts. The correct answer isn't listed here, but the user probably wants me to figure out the right answer and explain why.

First, I need to recall the cardiac conduction system. The heart's electrical activity starts in the sinoatrial (SA) node, which is the natural pacemaker. From there, the impulse spreads through the atria, causing them to contract. Then it reaches the atrioventricular (AV) node. The AV node delays the signal slightly, allowing the atria to finish contracting before the ventricles start. After the AV node, the impulse travels down the bundle of His, which splits into left and right bundle branches. These branches carry the impulse to the apex of the heart and then up the ventricular walls via the Purkinje fibers.

Wait, but the question is specifically about the depolarization of the ventricular muscle. The Purkinje fibers are part of the conduction system that spreads the impulse to the ventricles. So the depolarization of the ventricular muscle would start at the apex, right? Because the Purkinje fibers spread the impulse from the apex upward. So the ventricles depolarize from the apex up, leading to contraction from the bottom up. That makes sense because the blood is pushed upward into the aorta and pulmonary artery.

Now, looking at the options (even though they're not listed), the correct answer should be the apex. Let me check if there are other possibilities. The AV node is part of the conduction system but it's in the atria. The bundle of His is part of the pathway but doesn't directly cause ventricular depolarization. The Purkinje fibers are the ones that spread the impulse to the ventricular muscle. So the starting point would be the Purkinje fibers at the apex. But wait, the question says "starts from" the ventricular muscle. So the first point of depolarization in the ventricular muscle is the apex. Therefore, the correct answer is the apex of the heart.

Let me make sure there's no confusion with the SA node. The SA node is in the atria, so that's not the ventricular depolarization start. The AV node is in the atrial septum, so again, not the ventricles. The bundle branches are part of the conduction system but they lead to the Purkinje fibers. So the Purkinje fibers in the apex initiate the depolarization of the ventricular muscle. So the correct answer is the apex. Therefore, the option that mentions the apex would be correct. If the options include the apex, that's the right choice. The other options like SA node, AV node, or bundle of His would be incorrect because they're not the starting point for ventricular depolarization.

**Core Concept**
Depolarization of the ventricular myocardium begins at the **apex** of the heart, propagated via the **Purkinje fibers**, which are specialized conducting fibers in the subendocardium. This ensures a coordinated contraction from the apex upward, optimizing blood ejection.

**Why the Correct Answer is Right**
The cardiac conduction system directs ventricular depolarization via the **bundle of His → left/right bundle branches → Purkinje fibers**.

✓ Correct Answer: B. Left side of interventricular septum