**Core Concept**
Quinidine is a Class IA antiarrhythmic drug that affects the cardiac action potential by altering ion channel function. It prolongs the action potential duration (APD) and decreases the slope of Phase 0 depolarization, primarily by blocking potassium channels and to a lesser extent, sodium channels.

**Why the Correct Answer is Right**
Quinidine's mechanism of action involves the blockade of potassium channels (mainly I_Kr and I_Ks) and sodium channels (INa) in the cardiac myocyte. This blockade prolongs the time it takes for the potassium channels to repolarize the cell, thereby increasing the action potential duration. Additionally, quinidine's effect on sodium channels reduces the slope of Phase 0 depolarization, slowing the rate of rapid depolarization.

**Why Each Wrong Option is Incorrect**
**Option A:** Lignocaine (Lidocaine) is a Class IB antiarrhythmic that primarily blocks sodium channels, reducing the automaticity of the heart and shortening the action potential duration, not prolonging it.

**Option B:** Propanolol is a beta-blocker that primarily affects the heart rate by reducing the automaticity of the sinoatrial node and slowing conduction velocity through the AV node. It does not significantly affect the slope of Phase 0 or prolong the action potential duration.

**Option D:** Adenosine is a purinergic receptor agonist that transiently blocks AV nodal conduction, reducing heart rate. It does not affect the slope of Phase 0 or prolong the action potential duration.

**Clinical Pearl / High-Yield Fact**
Remember that Class IA antiarrhythmic drugs like quinidine can cause QT interval prolongation, which increases the risk of torsades de pointes, a potentially life-threatening arrhythmia.

**✓ Correct Answer: C. Quinidine**