**Core Concept**
Digoxin is a cardiac glycoside used to treat heart failure and atrial fibrillation. Its toxicity can be life-threatening and is often related to its narrow therapeutic index. The condition of a patient, particularly their kidney function, can affect digoxin levels and increase the risk of toxicity.

**Why the Correct Answer is Right**
Digoxin toxicity can be caused by an increase in its plasma levels, which can be due to decreased renal excretion, age, hypokalemia, hypomagnesemia, and hypercalcemia. These conditions can lead to an increased risk of toxicity by either increasing the drug's plasma concentration or affecting its binding to cardiac receptors.

**Why Each Wrong Option is Incorrect**
**Option A:**
* This is incorrect because age is a significant risk factor for digoxin toxicity.

**Option B:**
* This is incorrect because hypokalemia can increase the risk of digoxin toxicity by increasing the binding of digoxin to cardiac receptors.

**Option C:**
* This is incorrect because hypercalcemia can increase the risk of digoxin toxicity by increasing the binding of digoxin to cardiac receptors.

**Clinical Pearl / High-Yield Fact**
A classic exam trap is that hyperkalemia is not a risk factor for digoxin toxicity. In fact, hyperkalemia can protect against digoxin toxicity.

**Correct Answer:**
D.