Repolarization is due to opening of which channels

**Core Concept**
Repolarization in the cardiac action potential refers to the return of the membrane potential to its resting state. This process involves the closure of voltage-gated sodium channels and the opening of potassium channels. The potassium channels allow the efflux of potassium ions, which contributes to the repolarization of the cardiac cell.

**Why the Correct Answer is Right**
The primary channels responsible for repolarization are the potassium channels, specifically the slow delayed rectifier potassium channels (IKs) and the rapid delayed rectifier potassium channels (IKr). These channels are sensitive to voltage changes and open in response to depolarization, allowing potassium ions to flow out of the cell and back to the extracellular space. This outward flow of potassium ions helps to counteract the inward flow of calcium ions during the plateau phase of the action potential, ultimately leading to repolarization.

**Why Each Wrong Option is Incorrect**
**Option A:** Incorrect, as the opening of sodium channels is primarily responsible for depolarization, not repolarization.
**Option B:** Incorrect, as calcium channels are involved in the plateau phase of the action potential, but their opening does not contribute to repolarization.
**Option C:** Incorrect, as the opening of chloride channels is not a primary mechanism of repolarization in the cardiac action potential.

**Clinical Pearl / High-Yield Fact**
The repolarization phase of the cardiac action potential is crucial for the regulation of heart rate and rhythm. Abnormalities in potassium channel function can lead to arrhythmias, such as long QT syndrome, which can be life-threatening.

**Correct Answer: C. Potassium channels**