**Core Concept**
The therapeutic index (TI) or therapeutic range is a measure of the relative safety of a drug, calculated as the ratio of the toxic dose to the therapeutic dose. A narrow therapeutic range indicates that the difference between the effective dose and the toxic dose is small, making it challenging to achieve the desired therapeutic effect without causing adverse effects.

**Why the Correct Answer is Right**
The correct answer is a drug that requires careful monitoring of blood levels to avoid toxicity. This is because the therapeutic range is narrow, meaning that even small increases in dose can lead to toxic levels. The drug interacts with a specific enzyme or receptor, and its efficacy is closely tied to its concentration in the bloodstream. For example, drugs that affect ion channels or neurotransmitter systems may have a narrow therapeutic range.

**Why Each Wrong Option is Incorrect**
**Option A:** This drug has a wide therapeutic range, allowing for a relatively safe margin of error in dosing.
**Option B:** This drug is not typically associated with a narrow therapeutic range, and its toxicity is not as closely tied to blood levels.
**Option C:** This drug has a unique mechanism of action that does not rely on a narrow therapeutic range.

**Clinical Pearl / High-Yield Fact**
The therapeutic index is an important consideration when choosing a medication, and drugs with narrow therapeutic ranges require careful monitoring and dose adjustments to avoid toxicity.

**Correct Answer:** C. Warfarin. Warfarin has a narrow therapeutic range and requires regular monitoring of international normalized ratio (INR) to avoid bleeding complications.