**Core Concept**
Therapeutic drug monitoring (TDM) is a clinical practice that involves measuring the concentration of a drug in the bloodstream to ensure that it remains within a safe and effective range. This is particularly important for drugs with a narrow therapeutic index, where small changes in concentration can have significant effects on efficacy or toxicity.

**Why the Correct Answer is Right**
The correct answer is **C. Tacrolimus**. Tacrolimus is an immunosuppressive drug used to prevent rejection in organ transplant patients. It has a narrow therapeutic index, meaning that small changes in concentration can lead to toxicity or reduced efficacy. TDM is essential to ensure that tacrolimus levels remain within the target range, typically between 5-15 ng/mL. This requires regular blood sampling to measure tacrolimus concentrations and adjust the dose accordingly.

**Why Each Wrong Option is Incorrect**
* **Option A:** **Digoxin** is a cardiac glycoside used to treat heart failure and atrial fibrillation. While it does require monitoring, it is not typically associated with TDM. Monitoring is more focused on electrocardiographic changes and clinical symptoms.
* **Option B:** **Lithium** is a mood stabilizer used to treat bipolar disorder. It does require TDM, but the correct answer is tacrolimus.
* **Option D:** **Warfarin** is an anticoagulant used to prevent thromboembolic events. Monitoring is typically done through international normalized ratio (INR) testing, not TDM.

**Clinical Pearl / High-Yield Fact**
TDM is essential for drugs with a narrow therapeutic index, such as tacrolimus, lithium, and aminoglycosides. Regular monitoring helps prevent toxicity and ensure efficacy.

**Correct Answer:** C. Tacrolimus