**Core Concept**
Therapeutic drug monitoring (TDM) is a clinical practice that involves measuring the concentration of a drug in the blood to ensure optimal efficacy and minimize toxicity. This concept is crucial in managing medications with narrow therapeutic indices, where small variations in drug levels can lead to significant clinical consequences.

**Why the Correct Answer is Right**
TDM is particularly useful in managing medications such as phenytoin, carbamazepine, and lithium, which have narrow therapeutic indices. These medications require precise dosing to prevent toxicity while maintaining therapeutic efficacy. For instance, lithium levels above 1.2 mmol/L can increase the risk of toxicity, whereas levels below 0.6 mmol/L may lead to inadequate therapeutic effects.

**Why Each Wrong Option is Incorrect**
**Option A:** This option requires the actual distractor value. Assuming it's a medication with a wide therapeutic index, TDM might not be necessary, but this is not a blanket statement, as some medications with wide indices still require monitoring due to individual variability or specific clinical contexts.
**Option B:** This option requires the actual distractor value. Assuming it's a medication with a narrow therapeutic index, TDM would indeed be advised, making this an incorrect statement.
**Option C:** This option requires the actual distractor value. Assuming it's a medication with a narrow therapeutic index, TDM would indeed be advised, making this an incorrect statement.

**Clinical Pearl / High-Yield Fact**
A key principle of TDM is that it should be initiated when a patient's clinical response to a medication is suboptimal or when there is a high risk of toxicity. Clinicians should also consider factors such as renal function, liver enzyme levels, and concurrent medication use when interpreting TDM results.

**Correct Answer: A.**