**Question:** Mechanism of digitalis in atrial fibrillation:

A. Slows down the electrical conduction in the atria
B. Increases the contractility of the ventricles
C. Stimulates the vagal tone
D. Inhibits the sodium channels in the cardiac myocytes

**Correct Answer:** D. Inhibits the sodium channels in the cardiac myocytes

**Core Concept:** Digitalis is a cardiac glycoside that is used to treat heart failure and atrial fibrillation by modulating the electrical conduction and contractility of the heart.

**Why the Correct Answer is Right:** Digitalis works by inhibiting the sodium channels in cardiac myocytes, specifically the Nav1.5 sodium channel. This results in a slowing down of the electrical conduction in the atria, which reduces the rapid and chaotic conduction seen in atrial fibrillation. The slowing down of conduction helps in stabilizing the atrial rhythm and improving the ventricular response. Additionally, digitalis increases the contractility of the ventricles, improving the cardiac output.

**Why Each Wrong Option is Incorrect:**

A. Slowing down the electrical conduction in the atria is not the primary action of digitalis; it is the inhibition of sodium channels that is crucial for its effect.
B. While digitalis does increase ventricular contractility, the primary reason for its use in atrial fibrillation is to slow down the electrical conduction in the atria.
C. Stimulating vagal tone is not the primary action of digitalis; its primary effect is on sodium channels.

**Clinical Pearl:** Understanding the mechanism of action of digitalis is essential for its appropriate use in clinical practice. While digitalis may have minor effects on other cardiovascular parameters, its primary action is on sodium channels and electrical conduction in the atria, making it a crucial agent in managing atrial fibrillation and heart failure.