**Core Concept**
The absolute refractory period is a critical phase in the cardiac action potential where the heart muscle is completely incapable of generating a new action potential, regardless of the strength or duration of the stimulus. This period is characterized by a rapid depolarization of the cardiac myocyte membrane, leading to the opening of voltage-gated sodium channels and an influx of positively charged ions.

**Why the Correct Answer is Right**
The absolute refractory period is primarily due to the inactivation of sodium channels (Nav1.5) after the initial rapid depolarization phase (Phase 0) of the cardiac action potential. These channels, responsible for the initial influx of sodium ions, become inactivated and unavailable for reuse until the membrane potential returns to a more negative value, typically after the peak of Phase 1 (early repolarization). As a result, even strong stimuli cannot generate a new action potential during this period, ensuring proper cardiac rhythm and preventing premature contractions.

**Why Each Wrong Option is Incorrect**
* **Option A:** Incorrect because the absolute refractory period is not primarily due to potassium channels, which are activated during repolarization (Phase 3) and contribute to the repolarization process, but not the inactivation of sodium channels.
* **Option B:** Incorrect because the absolute refractory period is not due to calcium channels, which are primarily involved in the plateau phase (Phase 2) of the cardiac action potential and are not responsible for the inactivation of sodium channels.
* **Option D:** Incorrect because the absolute refractory period is not due to a delay in the repolarization process, but rather the inactivation of sodium channels, which makes the cardiac muscle unresponsive to stimuli.

**Clinical Pearl / High-Yield Fact**
The absolute refractory period is a critical concept in understanding cardiac arrhythmias, as disturbances in this period can lead to conditions such as tachyarrhythmias or fibrillation. It's essential to remember that the absolute refractory period occurs immediately after the peak of Phase 0 and lasts until the end of Phase 1, during which time the cardiac muscle is completely refractory to stimuli.

**Correct Answer:** C. Sodium channel inactivation