**Core Concept**
Digoxin toxicity can be caused by various factors, including drug interactions that alter digoxin levels or its pharmacokinetics. This question tests the student's knowledge of drugs that can increase digoxin levels, thereby increasing the risk of toxicity.

**Why the Correct Answer is Right**
Digoxin toxicity is often caused by drugs that increase digoxin levels by reducing its renal clearance or enhancing its absorption. Quinidine (Option A) is a known inhibitor of the P-glycoprotein efflux transporter, which increases digoxin levels. Hydrochlorothiazide (Option B) can increase digoxin levels by inducing hypokalemia, which in turn increases digoxin binding to cardiac tissues. Furosemide (Option D) can also increase digoxin levels by inducing hypokalemia and increasing renal digoxin reabsorption.

**Why Each Wrong Option is Incorrect**
**Option A:** Quinidine is a known inhibitor of the P-glycoprotein efflux transporter, which increases digoxin levels and increases the risk of toxicity.
**Option B:** Hydrochlorothiazide can increase digoxin levels by inducing hypokalemia, which in turn increases digoxin binding to cardiac tissues.
**Option D:** Furosemide can also increase digoxin levels by inducing hypokalemia and increasing renal digoxin reabsorption.

**Clinical Pearl / High-Yield Fact**
When managing patients on digoxin, it is essential to monitor potassium levels closely, as hypokalemia can increase the risk of digoxin toxicity.

**Correct Answer Line**
✓ Correct Answer: C. Triamterene. Triamterene is a potassium-sparing diuretic that does not increase digoxin levels or the risk of toxicity.