**Core Concept**
Serum creatinine levels are used to estimate glomerular filtration rate (GFR), which is a measure of kidney function. Patients with impaired kidney function, indicated by elevated serum creatinine levels, require adjustments in medication dosages to prevent toxicity.

**Why the Correct Answer is Right**
The correct answer involves medications that are primarily excreted by the kidneys. In patients with impaired kidney function, these medications can accumulate and lead to toxicity. Aminoglycoside antibiotics, such as gentamicin, are primarily excreted by the kidneys and require dosage reduction in patients with renal impairment. The kidneys play a crucial role in eliminating these antibiotics, and impaired kidney function can lead to prolonged half-lives and increased risk of toxicity.

**Why Each Wrong Option is Incorrect**
* **Option A:** Digitalis (e.g., digoxin) primarily depends on the liver for its metabolism and does not require dosage reduction in patients with renal impairment.
* **Option B:** Furosemide is a loop diuretic that is primarily excreted by the kidneys, but it does not require dosage reduction in patients with renal impairment; instead, it may be used to assess kidney function.
* **Option C:** Metformin is primarily excreted by the kidneys, but it does not require dosage reduction in patients with renal impairment; instead, it may be used cautiously in patients with kidney disease.

**Clinical Pearl / High-Yield Fact**
Patients with renal impairment often require adjustments in medication dosages, particularly for medications that are primarily excreted by the kidneys. It is essential to consider the patient's kidney function when selecting and dosing medications to prevent toxicity.

**Correct Answer:** C