**Core Concept**
Digitalis toxicity can manifest as various cardiac arrhythmias, including ventricular extrasystoles, in patients with heart failure (CHF) receiving digitalis therapy. The underlying mechanism involves enhanced automaticity and re-entry due to increased intracellular calcium and sodium levels.

**Why the Correct Answer is Right**
To counteract ventricular extrasystoles caused by digitalis toxicity, the potassium-sparing diuretic amiloride can be administered safely. Amiloride works by inhibiting the epithelial sodium channel (ENaC), which reduces sodium reabsorption and subsequently decreases intracellular sodium levels. Lower intracellular sodium levels decrease the influx of calcium ions through the sodium-calcium exchanger, thereby reducing the risk of digitalis-induced arrhythmias.

**Why Each Wrong Option is Incorrect**
**Option A:** Quinidine is a Class 1A antiarrhythmic agent that can further depress cardiac contractility and increase the risk of digitalis toxicity, making it an inappropriate choice for this patient.
**Option B:** Furosemide, a loop diuretic, can exacerbate hypokalemia, which is a known risk factor for digitalis toxicity and arrhythmias.
**Option C:** Nifedipine, a calcium channel blocker, can further decrease cardiac contractility and may worsen heart failure symptoms, making it an unsuitable choice for this patient.

**Clinical Pearl / High-Yield Fact**
In patients receiving digitalis therapy, potassium-sparing diuretics like amiloride can help prevent digitalis-induced arrhythmias by reducing intracellular sodium levels and subsequent calcium influx.

**Correct Answer:** C. Amiloride can be administered safely to counteract this arrhythmia.