**Core Concept**
The glomerular filtration rate (GFR) is a measure of kidney function that represents the volume of fluid filtered from the renal glomeruli per unit time. It is an essential indicator of renal health and function, and estimating GFR accurately is crucial for diagnosing and managing kidney disease.

**Why the Correct Answer is Right**
The most accurate method to estimate GFR is the modification of diet in renal disease (MDRD) equation, which takes into account the serum creatinine level, age, sex, and race of the individual. The MDRD equation is widely used in clinical practice due to its simplicity and high correlation with measured GFR. The equation is as follows: eGFR (mL/min/1.73m^2) = 186 × (serum creatinine)^-1.154 × (age)^-0.203 × 0.742 (if female) × 1.210 (if African American).

**Why Each Wrong Option is Incorrect**
* **Option A:** Serum urea levels are not a reliable indicator of GFR, as urea reabsorption and production rates can vary widely among individuals.
* **Option B:** The Cockcroft-Gault equation is another method to estimate GFR, but it is less accurate than the MDRD equation and is not recommended for use in patients with kidney disease.
* **Option D:** Measuring 24-hour urine creatinine clearance is a gold standard for estimating GFR, but it is impractical and time-consuming, making it unsuitable for routine clinical use.

**Clinical Pearl / High-Yield Fact**
When estimating GFR, it's essential to note that the MDRD equation is less accurate in patients with advanced kidney disease, and other methods such as iothalamate clearance may be more reliable in these cases.

**Correct Answer: C. MDRD equation. Modification of diet in renal disease (MDRD) equation.**