**Core Concept**
The MDRD formula is a widely used equation for estimating glomerular filtration rate (GFR) in patients with chronic kidney disease. It takes into account several variables to calculate an individual's GFR, which is essential for assessing kidney function.

**Why the Correct Answer is Right**
The MDRD formula primarily considers serum creatinine, age, gender, and race as its variables. These factors are used to calculate the GFR, which is a critical measure of kidney function. The formula is as follows: GFR (mL/min/1.73 m^2) = 186 × (serum creatinine)^-1.154 × (age)^-0.203 × (0.742 if female) × (1.210 if African American). Weight is not a variable in the MDRD formula, as it does not significantly affect the calculation of GFR.

**Why Each Wrong Option is Incorrect**
**Option A:** Age is a crucial variable in the MDRD formula, as it affects the calculation of GFR. It is used to adjust the formula's output to account for the decline in kidney function that occurs with increasing age.
**Option B:** Gender is also a variable in the MDRD formula, as females tend to have a lower muscle mass than males, which affects creatinine production and, subsequently, GFR calculation.
**Option C:** Race is a variable in the MDRD formula, as African Americans tend to have a higher muscle mass and, consequently, higher creatinine levels than non-African Americans. This difference is accounted for in the formula through the use of a correction factor.

**Clinical Pearl / High-Yield Fact**
When using the MDRD formula to estimate GFR, it is essential to note that the formula is not suitable for patients with acute kidney injury, as it does not accurately reflect the rapid changes in kidney function that occur in this condition.

**Correct Answer:**
✓ Correct Answer: D. Weight