**Core Concept**
Therapeutic drug monitoring (TDM) is essential for medications with a narrow therapeutic index, where small variations in drug concentration can significantly impact efficacy or toxicity. Gentamicin, an aminoglycoside antibiotic, falls into this category due to its potential for nephrotoxicity and ototoxicity at higher concentrations.

**Why the Correct Answer is Right**
Gentamicin requires TDM because its plasma concentrations can fluctuate significantly, especially with once-daily dosing regimens. This is due to its concentration-dependent pharmacokinetics, where higher concentrations are more likely to cause renal damage. Monitoring gentamicin levels helps prevent toxicity while ensuring adequate bactericidal concentrations against target pathogens. The therapeutic range for gentamicin is typically considered to be between 4-12 mg/L.

**Why Each Wrong Option is Incorrect**
**Option A:** Sulfonamides are generally considered safe and do not require TDM, except in specific cases of sulfonamide allergy or hypersensitivity. They have a relatively wide therapeutic index.
**Option B:** Metformin is a biguanide antidiabetic agent that does not require TDM, as its efficacy and safety are not significantly affected by plasma concentrations. Metformin's pharmacokinetics are linear, and it is not associated with significant toxicity at therapeutic doses.
**Option C:** Cycloserine, an antibiotic used for tuberculosis treatment, does not typically require TDM. While it can cause neurotoxicity, this is more related to dose and duration of therapy rather than plasma concentration.

**Clinical Pearl / High-Yield Fact**
Aminoglycosides like gentamicin require careful monitoring and dose adjustment, especially in patients with renal impairment or those receiving other nephrotoxic agents.

**✓ Correct Answer: D. Gentamycin**