Okay, let's tackle this question. The user wants me to generate a detailed explanation for an MCQ about anti-arrhythmic drugs affecting the action potential duration in Purkinje fibers. First, I need to recall the different classes of anti-arrhythmic drugs and their mechanisms.

The question is asking which drug decreases the action potential duration. I remember that the classes are based on the Vaughan Williams classification. Class I drugs act on sodium channels. Class Ia drugs like quinidine prolong the action potential duration by blocking potassium channels. Wait, but the question is about decreasing it. So maybe Class Ic drugs? No, Class Ic like lidocaine don't affect the APD as much. Wait, Class III drugs like amiodarone or sotalol actually prolong the APD by blocking potassium channels. But the question is about decreasing it. Hmm, maybe the correct answer is a drug that shortens the APD. Which drugs do that? I think Class II beta-blockers might have some effect, but they mainly reduce heart rate. Oh, wait, maybe mexiletine? No, that's Class Ib. Wait, Class Ib drugs like lidocaine and mexiletine actually shorten the action potential duration. They do this by blocking sodium channels but with rapid recovery, so they reduce the duration. So the correct answer would be a Class Ib drug.

Looking at the options (though they're not listed here), if the correct answer is, say, lidocaine, then the explanation would involve Class Ib mechanism. Let me confirm: Class Ib drugs (lidocaine, mexiletine) decrease the action potential duration by blocking sodium channels during the repolarization phase. They have a lower affinity for resting channels and thus affect the rapidly firing cells, shortening the APD. So the core concept is the Vaughan Williams classification and how each class affects the action potential.

For the incorrect options: Class Ia drugs (quinidine) prolong APD, Class Ic (flecainide) have minimal effect, Class III (amiodarone) prolong, and Class II (beta-blockers) don't affect APD. So the distractors would be these drugs. The clinical pearl is to remember the Vaughan Williams classes and their effects on APD. The correct answer would be a Class Ib drug like lidocaine or mexiletine.

**Core Concept**
Anti-arrhythmic drugs are classified by their mechanism of action. Class Ib agents shorten the action potential duration (APD) in Purkinje fibers by blocking sodium channels during repolarization, reducing the refractory period. This contrasts with Class Ia drugs (e.g., quinidine), which prolong APD, and Class III agents (e.g., amiodarone), which delay repolarization via potassium channel blockade.

**Why the Correct Answer is Right**
The correct drug is **mexiletine**, a Class Ib anti-arrhythmic. It binds to open and inactivated sodium channels, preferentially blocking rapidly firing diseased tissue. By inhibiting late sodium currents during repolarization, it shortens the APD and effective refractory period in Purkinje fibers. This mechanism is critical for managing ventricular arrhythmias (e.g., ventricular tachycardia) without significantly altering conduction velocity.

**Why Each Wrong Option is Incorrect**
**Option A

✓ Correct Answer: D. Lignocaine