**Core Concept**
The question is testing the understanding of the mechanism of catecholamine-induced arrhythmias and the role of certain agents in sensitizing the myocardium to catecholamines. **Digitalis** is a cardiac glycoside that increases the sensitivity of the heart to catecholamines by inhibiting the sodium-potassium ATPase pump, leading to an increase in intracellular sodium and calcium.

**Why the Correct Answer is Right**
Digitalis increases the sensitivity of the myocardium to catecholamines by increasing the intracellular calcium levels, which enhances the contractility of the heart. This is achieved by inhibiting the sodium-potassium ATPase pump, leading to an increase in intracellular sodium, which in turn increases the activity of the sodium-calcium exchanger, resulting in an increase in intracellular calcium. The increased intracellular calcium sensitizes the myocardium to catecholamines, making it more susceptible to arrhythmias.

**Why Each Wrong Option is Incorrect**
* **Option A:** This option is incorrect because it is not directly related to the mechanism of sensitizing the myocardium to catecholamines.
* **Option B:** This option is incorrect because it is a beta-blocker, which actually decreases the sensitivity of the myocardium to catecholamines.
* **Option D:** This option is incorrect because it is a calcium channel blocker, which actually decreases the intracellular calcium levels and would not sensitize the myocardium to catecholamines.

**Clinical Pearl / High-Yield Fact**
Digitalis toxicity can be precipitated by administering catecholamines to a patient taking digitalis, highlighting the importance of monitoring patients for digitalis toxicity, especially in the setting of catecholamine administration.

**Correct Answer: C. Digitalis**