**Core Concept**
A drug with a narrow therapeutic range has a small difference between its effective dose and toxic dose. This means that small variations in plasma concentrations can lead to either inadequate efficacy or toxicity. Drugs with narrow therapeutic ranges often require careful dose titration and monitoring to avoid adverse effects.

**Why the Correct Answer is Right**
Digoxin, a cardiac glycoside, is a classic example of a medication with a narrow therapeutic range. Its mechanism of action involves inhibiting the sodium-potassium ATPase pump in cardiac cells, which increases the force of cardiac contractions. However, even small increases in digoxin levels can lead to toxic effects, such as arrhythmias and cardiac arrest. The therapeutic window for digoxin is approximately 0.5-2 ng/mL, making it a challenging medication to manage.

**Why Each Wrong Option is Incorrect**
**Option A:** Warfarin has a narrow therapeutic range, but it is an anticoagulant, not a classic example of a medication with a narrow therapeutic range like digoxin.

**Option B:** This option is blank, so I'll move on to the next one.

**Option C:** This option is also blank, so I'll explain why the correct answer is right and provide a clinical pearl.

**Clinical Pearl / High-Yield Fact**
When prescribing medications with narrow therapeutic ranges, it's essential to monitor plasma concentrations regularly to avoid toxicity. This is particularly important for medications like digoxin, where small changes in dose can have significant consequences.

**Correct Answer: D. Digoxin**