**Core Concept**
Quinidine is a class IA anti-arrhythmic medication that primarily affects the cardiac action potential. It works by blocking certain ion channels in the heart, leading to a prolongation of the action potential duration and a reduction in the rate of rise of the action potential.

**Why the Correct Answer is Right**
Quinidine primarily blocks the potassium (K+) channels in the heart, specifically the rapid component of the delayed rectifier potassium channel (IKr). This blockade prolongs the repolarization phase of the cardiac action potential, which can help to prevent or treat certain types of arrhythmias. By blocking potassium channels, quinidine also reduces the automaticity of the heart and slows conduction through the cardiac tissue.

**Why Each Wrong Option is Incorrect**
**Option A:** Ca+ Channel blocker - Quinidine does not primarily act as a calcium channel blocker; it does not affect the L-type calcium channels (Cav1.2) in the heart.
**Option C:** Na+ Channel opener - Quinidine actually blocks sodium channels, not opens them; it reduces the rate of rise of the action potential and slows conduction through the cardiac tissue.
**Option D:** Cl+ Channel opener - Quinidine does not primarily act on chloride channels; its primary mechanism of action is the blockade of potassium channels.

**Clinical Pearl / High-Yield Fact**
Quinidine is a classic example of a class IA anti-arrhythmic medication that can cause a prolongation of the QT interval. This can increase the risk of torsades de pointes, a type of polymorphic ventricular tachycardia. Patients taking quinidine should be closely monitored for signs of QT interval prolongation and arrhythmias.

**✓ Correct Answer: B. Kt Channel blocker Quinidine primarily blocks potassium channels in the heart.**