**Core Concept**
Therapeutic drug monitoring (TDM) is the clinical practice of measuring the concentration of a specific drug in a patient's bloodstream to optimize the efficacy and safety of the therapy. This practice is essential for drugs with a narrow therapeutic index, where small variations in plasma concentrations can lead to either ineffectiveness or toxicity.

**Why the Correct Answer is Right**
TDM is typically used for drugs with a narrow therapeutic index, such as antiepileptics (e.g., phenytoin, carbamazepine), anticoagulants (e.g., warfarin), and certain antibiotics (e.g., aminoglycosides). The goal of TDM is to maintain the plasma concentration within a target range to prevent both under-treatment and over-treatment.

**Why Each Wrong Option is Incorrect**
* **Option A:** This option is incorrect because TDM is not typically used for drugs with a wide therapeutic index, such as acetaminophen or aspirin, where the risk of toxicity is relatively low at therapeutic doses.
* **Option B:** This option is incorrect because while TDM may be used for certain immunosuppressants, it is not the primary indication for this practice.
* **Option C:** This option is incorrect because TDM is not typically used for drugs with a very short half-life, such as lorazepam or oxazepam, where the plasma concentration changes rapidly and is often not clinically relevant.

**Clinical Pearl / High-Yield Fact**
A mnemonic to remember the main indication for TDM is: "Elderly patients, Epileptics, and Exotics" (EPE), which stands for the groups of patients who commonly require TDM due to the narrow therapeutic index of their medications.

**Correct Answer: A. This option is incorrect because TDM is not typically used for drugs with a wide therapeutic index.**