**Core Concept**
The question is testing the concept of catecholamine-induced arrhythmias and the role of sensitizing agents in the myocardium. **Catecholamines** such as adrenaline and noradrenaline can increase heart rate and contractility, but certain agents can make the myocardium more susceptible to their arrhythmogenic effects.

**Why the Correct Answer is Right**
The correct answer is a **digitalis glycoside** (e.g., digoxin). Digitalis glycosides increase the sensitivity of the myocardium to catecholamines by increasing the density of beta-adrenergic receptors and enhancing the intracellular signaling pathways that lead to increased contractility. This can lead to arrhythmias, particularly in the presence of high levels of catecholamines.

**Why Each Wrong Option is Incorrect**
* **Option A:** This option is not relevant to the question.
* **Option B:** This option is incorrect because it is not a known sensitizing agent for catecholamines.
* **Option D:** This option is incorrect because it is actually used to block the effects of catecholamines, not sensitize the myocardium.

**Clinical Pearl / High-Yield Fact**
Digitalis toxicity can be induced or exacerbated by high levels of catecholamines, which is why patients on digitalis should be monitored closely in situations where catecholamines are being administered, such as during stress tests or in the presence of certain medical emergencies.

**Correct Answer:** C. Digitalis glycoside