**Core Concept**
The Purkinje fibers are a critical part of the cardiac conduction system, responsible for transmitting electrical impulses from the atrioventricular node to the ventricular muscle. The stimulation of Purkinje fibers triggers a coordinated contraction of the ventricles, ensuring efficient cardiac output.

**Why the Correct Answer is Right**
When the Purkinje fibers are stimulated, the electrical impulse travels from the septum, which is the muscular wall separating the left and right ventricles. The impulse then spreads to the endocardium, the innermost layer of the heart, before finally reaching the epicardium, the outermost layer. This order of activation is crucial for ensuring a synchronized contraction of the ventricular muscle. The septum serves as the initial site of activation, followed by the endocardium, and finally the epicardium. This sequence of activation allows for a coordinated contraction of the ventricular muscle, ensuring efficient cardiac output.

**Why Each Wrong Option is Incorrect**
**Option B:** Incorrect because the endocardium is not the initial site of activation. The septum serves as the starting point for the electrical impulse.
**Option C:** Incorrect because the epicardium is not the site of initial activation. The impulse travels from the septum to the endocardium before reaching the epicardium.
**Option D:** Incorrect because the epicardium is not activated before the endocardium. The correct order is septum → endocardium → epicardium.

**Clinical Pearl / High-Yield Fact**
It's essential to remember that the Purkinje fibers play a crucial role in the cardiac conduction system, ensuring a coordinated contraction of the ventricular muscle. The order of activation (septum → endocardium → epicardium) is critical for maintaining efficient cardiac output.

**✓ Correct Answer: A. Septum → Endocardium → Epicardium**