**Core Concept**
Digoxin is a cardiac glycoside used to treat heart failure and atrial fibrillation. Its plasma concentration can reach toxic levels in certain patient populations, leading to digitalis toxicity. This condition is characterized by symptoms such as nausea, vomiting, visual disturbances, and cardiac arrhythmias.

**Why the Correct Answer is Right**
Digoxin is primarily excreted by the kidneys. Patients with impaired renal function have reduced clearance of digoxin, leading to its accumulation in the body. This is because the kidneys are responsible for filtering and excreting the drug, and in the case of renal failure, this process is impaired. As a result, the plasma concentration of digoxin increases, potentially leading to toxicity.

**Why Each Wrong Option is Incorrect**
**Option A:** While hypokalemia can increase the risk of digitalis toxicity, it is not the primary reason for digoxin accumulation.
**Option B:** Certain medications, such as quinidine, can increase the risk of digitalis toxicity, but they do not directly contribute to digoxin accumulation.
**Option C:** Elderly patients may have reduced renal function, but this is not the most significant factor in digoxin accumulation.

**Clinical Pearl / High-Yield Fact**
In patients with renal impairment, it is essential to monitor digoxin levels closely and adjust the dosage accordingly to prevent toxicity.

**Correct Answer:** C. Elderly patients