**Core Concept**
Therapeutic drug monitoring (TDM) is a medical practice that involves measuring the concentration of drugs in the bloodstream to ensure they are within a safe and effective range. This is particularly important for medications with a narrow therapeutic index, where small changes in drug concentration can lead to toxicity or reduced efficacy.

**Why the Correct Answer is Right**
The correct answer is **Digoxin**. This is because digoxin has a narrow therapeutic index and its toxicity can be life-threatening. TDM is essential to prevent digoxin toxicity, which can occur even at therapeutic doses. The mechanism involves measuring the concentration of digoxin in the blood, which is typically done 6-8 hours after administration. The ideal range is between 0.8-2.0 ng/mL.

**Why Each Wrong Option is Incorrect**
* **Option B:** **Lithium** is also a medication that requires TDM, but it is not the correct answer in this context. Lithium has a narrow therapeutic index and its toxicity can cause serious side effects, but it is not the most common medication requiring TDM.
* **Option C:** **Warfarin** is an anticoagulant that requires regular monitoring of its International Normalized Ratio (INR), but it does not require TDM in the same way as digoxin.
* **Option D:** **Metformin** is an oral antidiabetic medication that does not require TDM. Its efficacy and safety are monitored through clinical assessment and regular blood glucose measurements.

**Clinical Pearl / High-Yield Fact**
A memorable point about TDM is that it is essential for medications with a narrow therapeutic index, such as digoxin and lithium. Regular monitoring of these medications can prevent serious side effects and ensure optimal efficacy.

**Correct Answer:** D. Warfarin is not the best option but Metformin is not correct for TDM.