**Core Concept**
The action potential duration in Purkinje fibers is a critical parameter in cardiac electrophysiology. Anti-arrhythmic drugs can modulate this duration, influencing the heart's rhythm. Lignocaine (Lidocaine) is a class Ib anti-arrhythmic agent that affects cardiac ion channels.

**Why the Correct Answer is Right**
Lignocaine decreases the action potential duration in Purkinje fibers by blocking the sodium channels, which are responsible for the rapid depolarization phase of the action potential. This blockade reduces the influx of sodium ions, thereby shortening the action potential duration. Lignocaine's primary mechanism of action is to inhibit the sodium channels, which are more prevalent in the Purkinje fibers and ventricular muscle cells.

**Why Each Wrong Option is Incorrect**
**Option A:** Quinidine is a class Ia anti-arrhythmic agent that prolongs the action potential duration by blocking potassium channels and sodium channels, but it does not decrease the action potential duration.

**Option B:** Flecainide is a class Ic anti-arrhythmic agent that blocks sodium channels, but it does not decrease the action potential duration. Instead, it prolongs the action potential duration by slowing the conduction velocity.

**Option C:** Amiodarone is a class III anti-arrhythmic agent that prolongs the action potential duration by blocking potassium channels, which increases the duration of the action potential.

**Clinical Pearl / High-Yield Fact**
Lignocaine is a fast-acting anti-arrhythmic agent that is often used to treat ventricular arrhythmias, but it has a short half-life and requires frequent dosing to maintain its effects.

**✓ Correct Answer: D. Lignocaine**