## **Core Concept**
The classification of antiarrhythmic drugs into classes I, II, III, IV, and V is based on their primary mechanism of action. This system is widely used to categorize these medications according to their effects on the cardiac action potential. The Vaughan-Williams classification is the most commonly used system for this purpose.

## **Why the Correct Answer is Right**
The Vaughan-Williams classification system categorizes antiarrhythmic drugs as follows:
- Class I: Sodium channel blockers, further divided into:
- **Class Ia**: Prolongs the action potential (e.g., quinidine, procainamide).
- **Class Ib**: Shortens the action potential (e.g., lidocaine, mexiletine).
- **Class Ic**: Minimal effect on the action potential duration (e.g., flecainide, propafenone).
- Class II: Beta-blockers.
- Class III: Potassium channel blockers, prolonging the action potential (e.g., amiodarone, sotalol).
- Class IV: Calcium channel blockers.
- Class V: Includes drugs that work through other or miscellaneous mechanisms.

## **Why Each Wrong Option is Incorrect**
- **Option A**: Not specified, but if not Vaughan-Williams, then it's incorrect.
- **Option B**: Not specified, but any other classification system or incorrect attribution would make it wrong.
- **Option C**: Similarly, without specifics, any incorrect classification system or attribution makes it incorrect.

## **Clinical Pearl / High-Yield Fact**
A key point to remember is that the Vaughan-Williams classification, while useful, does not encompass all the complexities of drug action, and some drugs may have properties from multiple classes. For example, amiodarone has actions of classes I, II, III, and IV.

## **Correct Answer:** . Vaughan-Williams.