**Core Concept**
The MDRD formula is a widely used equation to estimate the glomerular filtration rate (GFR) in patients with chronic kidney disease. It takes into account serum creatinine levels, age, sex, and race to calculate an estimated GFR (eGFR).

**Why the Correct Answer is Right**
The MDRD formula uses the following variables: serum creatinine, age, sex, and race. It does not require the measurement of blood urea nitrogen (BUN) or actual urine creatinine levels to estimate GFR. This makes it a more practical and accessible tool for clinical use. The formula was derived from the Modification of Diet in Renal Disease study, which aimed to understand the relationship between dietary protein intake and kidney function in patients with chronic kidney disease.

**Why Each Wrong Option is Incorrect**
**Option A:** Not applicable, as the question asks which variable is not included in the MDRD formula.
**Option B:** BUN is not a required variable in the MDRD formula, but it is often used as a companion test to assess kidney function.
**Option C:** Actual urine creatinine levels are not required in the MDRD formula, making it a more straightforward and practical tool for estimating GFR.
**Option D:** Age, sex, and race are all included in the MDRD formula, making this option incorrect.

**Clinical Pearl / High-Yield Fact**
Remember that the MDRD formula is a simplified estimate of GFR and should be used in conjunction with clinical judgment and other diagnostic tests to accurately diagnose and manage kidney disease.

**Correct Answer:** B. BUN.