**Core Concept**
Creatinine clearance is a measure of the glomerular filtration rate (GFR), which is an indicator of kidney function. It estimates the volume of blood plasma that is cleared of creatinine per unit time, providing a useful index of renal function.

**Why the Correct Answer is Right**
Creatinine clearance is calculated using the Cockcroft-Gault formula or the MDRD (Modification of Diet in Renal Disease) equation, which takes into account the serum creatinine level, age, sex, weight, and other factors. The result is expressed in milliliters per minute (mL/min) or liters per minute (L/min) and is used to diagnose and monitor kidney disease, including chronic kidney disease (CKD) and acute kidney injury (AKI). The GFR is a critical parameter in assessing the severity of kidney dysfunction and guiding treatment decisions.

**Why Each Wrong Option is Incorrect**
**Option A:** This option is not relevant to the assessment of kidney function.
**Option B:** While serum creatinine is used to estimate GFR, it is not a measure of kidney function itself.
**Option C:** Creatinine clearance is not used to assess liver function.

**Clinical Pearl / High-Yield Fact**
In clinical practice, a creatinine clearance of < 60 mL/min is often used as a threshold to diagnose CKD. This value indicates a reduced GFR, which may lead to complications such as electrolyte imbalances, fluid overload, and accelerated cardiovascular disease. **Correct Answer: D. Glomerular filtration rate (GFR)**