**Core Concept**
Digitalis, a cardiac glycoside, is used to treat heart failure and arrhythmias by increasing the contractility of the heart and slowing the heart rate. The therapeutic effect of digitalis is influenced by various factors, including its interaction with other medications.

**Why the Correct Answer is Right**
The correct answer is **A. Quinidine**. Quinidine, an antiarrhythmic medication, increases the concentration of digitalis in the body by inhibiting its metabolism in the liver. This leads to a synergistic effect, where the combination of quinidine and digitalis can cause a significant increase in the risk of digitalis toxicity. Quinidine also affects the potassium channels in the heart, which can lead to a further increase in the sensitivity of the heart to digitalis.

**Why Each Wrong Option is Incorrect**
**Option B:** Amiodarone is another antiarrhythmic medication, but it does not interact with digitalis in the same way as quinidine. Amiodarone can actually decrease the concentration of digitalis in the body by increasing its clearance.

**Option C:** Furosemide, a loop diuretic, can actually decrease the concentration of digitalis in the body by increasing its clearance and reducing its reabsorption.

**Option D:** Verapamil, a calcium channel blocker, can interact with digitalis to increase its effect on the heart, but it does not interact with digitalis as maximally as quinidine.

**Clinical Pearl / High-Yield Fact**
When prescribing digitalis, it's essential to consider the patient's underlying medical conditions, such as heart failure and arrhythmias, as well as their medication regimen, including any potential interactions with other medications like quinidine.

**Correct Answer: A. Quinidine**