**Question:** Mexiletine is an oral active congener of

A. Class Ic antiarrhythmic agents
B. Class III antiarrhythmic agents
C. Calcium channel blockers
D. Sodium channel blockers

**Correct Answer:** D. Sodium channel blockers

**Core Concept:**
Mexiletine is a medication used for the treatment of ventricular arrhythmias, particularly those associated with the Wolff-Parkinson-White (WPW) syndrome and ventricular tachycardia. It is classified under the broader category of antiarrhythmic agents.

**Why the Correct Answer is Right:**
Mexiletine is classified as a sodium channel blocker, more specifically, it is a class Ic antiarrhythmic agent. Class Ic drugs act by blocking voltage-gated sodium channels in cardiac myocytes, which results in a decrease in ventricular excitability and refractoriness, ultimately reducing the likelihood of reentrant arrhythmias.

**Why Each Wrong Option is Incorrect:**
A. Class Ic drugs, like mexiletine, do not share the same mechanism of action as class III antiarrhythmic agents (e.g., sotalol), which primarily focus on prolonging the action potential in the ventricles.
B. Calcium channel blockers (e.g., diltiazem, verapamil) are drugs that affect calcium ions and their interaction with cardiac muscle cells, not sodium channels, which is the target of mexiletine.
C. Mexiletine is not related to calcium channel blockers, as it belongs to a different class of drugs targeting voltage-gated sodium channels. Calcium channel blockers, on the other hand, are primarily used for hypertension and angina, not specifically for arrhythmias.

**Clinical Pearl:**
Mexiletine and lidocaine are often used together in the treatment of WPW syndrome and ventricular tachycardia due to their complementary mechanisms of action. While mexiletine targets voltage-gated sodium channels, lidocaine targets voltage-gated calcium channels, thereby providing a broader range of action in suppressing reentrant arrhythmias.

Mexiletine is a sodium channel blocker, specifically targeting voltage-gated sodium channels in cardiac myocytes. By blocking these channels, mexiletine reduces the excitability and refractoriness of the ventricles, which helps to suppress reentrant arrhythmias commonly seen in WPW syndrome and ventricular tachycardia.