**Core Concept**
The question is testing knowledge of the cardiac toxicity of various medications, specifically their association with cardiac mortality. This requires understanding the pharmacological properties of different drugs, their effects on the cardiovascular system, and the potential risks they pose to cardiac health.

**Why the Correct Answer is Right**
The correct answer is **C. Quinidine**. Quinidine is an antiarrhythmic medication that belongs to the class IA category. It has a narrow therapeutic window and can cause severe cardiac toxicity, including torsades de pointes, ventricular fibrillation, and cardiac arrest. This is due to its ability to prolong the QT interval, increase the risk of arrhythmias, and cause myocardial depression. Quinidine's cardiac toxicity is a well-documented concern, and it has been associated with a significant increase in cardiac mortality.

**Why Each Wrong Option is Incorrect**
* **Option A:** This option is incorrect because while other antiarrhythmic medications may also cause cardiac toxicity, quinidine has the highest association with cardiac mortality.
* **Option B:** This option is incorrect because digoxin, an antiarrhythmic medication, primarily affects the atrioventricular node and has a different mechanism of action compared to quinidine.
* **Option D:** This option is incorrect because beta-blockers, such as propranolol, are generally considered safe and are not associated with a high risk of cardiac mortality.

**Clinical Pearl / High-Yield Fact**
Quinidine's cardiac toxicity is often related to its ability to block potassium channels, leading to QT interval prolongation and increased risk of arrhythmias. This highlights the importance of monitoring patients taking quinidine for cardiac toxicity and adjusting doses carefully to minimize risks.

**Correct Answer: C. Quinidine**