**Question:** Mechanism of action of disopyramide is

A. Inhibition of sodium channels
B. Inhibition of calcium channels
C. Blockade of voltage-gated calcium channels
D. Preferential inhibition of cardiac sodium channels

**Core Concept:** Disopyramide is a class Ic antiarrhythmic drug that is mainly used in the treatment of ventricular arrhythmias, particularly those with ectopic foci. It works by inhibiting voltage-gated sodium channels, which leads to a reduction in the availability of sodium ions at the cardiac myocyte membrane, ultimately slowing down the depolarization process and decreasing the excitability of the cardiac tissue.

**Why the Correct Answer is Right:** Disopyramide primarily targets cardiac sodium channels, which are involved in the depolarization phase of the cardiac action potential. By blocking these channels, it reduces the availability of sodium ions at the cardiac myocyte membrane. This leads to a slowing down of the depolarization process and reduces cardiac tissue excitability, thereby exerting its antiarrhythmic effect.

**Why Each Wrong Option is Incorrect:**

A. Inhibition of sodium channels is partially correct, but disopyramide's main mechanism lies in its selective targeting of cardiac sodium channels, making option D the correct answer.
B. Inhibition of calcium channels is incorrect as disopyramide primarily affects sodium channels and has no significant impact on calcium channels.
C. Blockade of voltage-gated calcium channels is incorrect because disopyramide's primary action is on sodium channels, not calcium channels.

**Clinical Pearl:** The preferential inhibition of cardiac sodium channels by disopyramide contributes to its effectiveness in treating ventricular arrhythmias. However, it should be used cautiously in patients with impaired renal function due to the risk of neurotoxicity, and it is contraindicated in patients with severe bradycardia or third-degree heart block.