**Core Concept**
Atrioventricular (AV) block is a condition where the electrical conduction between the atria and ventricles is disrupted, leading to a delay or failure of the ventricles to contract in response to atrial activity. This can occur due to various factors, including digitalis toxicity, electrolyte imbalances, and certain cardiac conditions.

**Why the Correct Answer is Right**
Digitalis toxicity, particularly when accompanied by potassium (K+) depletion, can cause AV block by interfering with the normal functioning of the AV node. Digitalis, a cardiac glycoside, inhibits the Na+/K+-ATPase pump, leading to increased intracellular sodium and calcium levels. This can cause the AV node to become hyperexcitable, eventually leading to AV block. Potassium depletion exacerbates this effect by further increasing the intracellular calcium levels, thereby increasing the risk of AV block.

**Why Each Wrong Option is Incorrect**
**Option A:** WPW syndrome is characterized by an accessory electrical pathway between the atria and ventricles, leading to a pre-excitation of the ventricles. This results in a widened QRS complex and a short PR interval, but it is not typically associated with AV block.

**Option B:** Pacemaker complications can include infection, lead fracture, or malfunction, but AV block is not a direct complication of pacemaker therapy.

**Option C:** AV block can occur in individuals with underlying heart disease, but it is not a common occurrence in healthy individuals without any cardiac conditions.

**Clinical Pearl / High-Yield Fact**
Digitalis toxicity can masquerade as a variety of cardiac conditions, including AV block, atrial fibrillation, and ventricular tachycardia. It is essential to consider digitalis toxicity in the differential diagnosis of any patient with these symptoms, particularly in the context of recent digitalis therapy or electrolyte imbalances.

**Correct Answer Line**
✓ Correct Answer: D. Seen in Digitalis toxicity with K+ depletion