**Core Concept**
Digitalis toxicity is a well-known side effect of excessive digoxin therapy, especially in patients with pre-existing cardiac conditions. The condition is characterized by a complex interplay of effects on the cardiovascular system, including alterations in heart rate, contractility, and rhythm.

**Why the Correct Answer is Right**
The most characteristic arrhythmia associated with digitalis toxicity is a **bigeminy pattern**, specifically a sequence of normal beats followed by a premature ventricular contraction (PVC). This occurs due to the increased automaticity of ventricular muscle cells, which is a result of the inhibition of the Na+/K+-ATPase pump by digoxin. This leads to an increase in intracellular calcium, which in turn enhances the contractility of the heart muscle but also increases the likelihood of arrhythmias. The bigeminy pattern is a result of the alternating normal and PVC beats, which is a hallmark of digitalis toxicity.

**Why Each Wrong Option is Incorrect**
* **Option A:** While atrial flutter is a potential side effect of digitalis toxicity, it is not the most characteristic arrhythmia associated with this condition.
* **Option B:** Ventricular tachycardia is a serious arrhythmia, but it is not the most characteristic arrhythmia associated with digitalis toxicity.
* **Option D:** Atrial fibrillation is another potential side effect of digitalis toxicity, but it is not as specific or characteristic as the bigeminy pattern.

**Clinical Pearl / High-Yield Fact**
To remember the characteristic arrhythmia of digitalis toxicity, recall the phrase "Normal beats followed by a PVC ( Premature Ventricular Contraction ) - Big E ( Enhanced ) contractility of the heart muscle".