**Core Concept**
Digitalis, a cardiac glycoside, is used to treat atrial fibrillation by increasing the contractility of the heart and slowing the ventricular rate. This is achieved through its effects on the cardiac conduction system and the ion channels within the heart muscle cells.

**Why the Correct Answer is Right**
The mechanism of action of digitalis in atrial fibrillation involves the inhibition of the Na+/K+-ATPase pump, which leads to an increase in intracellular sodium levels. This increase in sodium levels causes an increase in intracellular calcium levels through the sodium-calcium exchanger. The increased intracellular calcium levels enhance the contractility of the heart muscle cells, resulting in a stronger cardiac output. Additionally, digitalis increases the refractory period of the atrioventricular (AV) node, which slows the ventricular rate and helps to control the symptoms of atrial fibrillation.

**Why Each Wrong Option is Incorrect**
* **Option A:** This option is incorrect because it does not accurately describe the mechanism of action of digitalis in atrial fibrillation. While digitalis does have some effect on the parasympathetic nervous system, its primary mechanism in treating atrial fibrillation is through the inhibition of the Na+/K+-ATPase pump.
* **Option B:** This option is incorrect because it describes the mechanism of action of beta-blockers, not digitalis. Beta-blockers work by blocking the effects of epinephrine on the heart, leading to a decrease in heart rate and contractility.
* **Option C:** This option is incorrect because it describes the mechanism of action of calcium channel blockers, not digitalis. Calcium channel blockers work by blocking the flow of calcium ions into the cardiac muscle cells, leading to a decrease in contractility and heart rate.

**Clinical Pearl / High-Yield Fact**
In patients with atrial fibrillation, digitalis should be used with caution due to the risk of digitalis toxicity, which can occur when the levels of digitalis in the blood become too high. Signs of digitalis toxicity include nausea, vomiting, and changes in heart rhythm.

**Correct Answer:** C.