**Core Concept**
Azotemia refers to an elevation of blood urea nitrogen (BUN) and creatinine levels, indicating impaired renal function. The glomerular filtration rate (GFR) is a key indicator of kidney function. Azotemia is a sign of decreased GFR.

**Why the Correct Answer is Right**
Azotemia occurs when the kidneys are not able to filter waste products effectively, leading to their accumulation in the blood. A residual GFR of less than 50% of normal is often associated with azotemia. This is because the kidneys' ability to filter waste is significantly impaired at this level.

**Why Each Wrong Option is Incorrect**
**Option A:** A residual GFR of 50-75% is generally not associated with azotemia, as the kidneys can still filter waste products relatively effectively.
**Option B:** A residual GFR of 75-100% indicates near-normal kidney function, and azotemia would not be expected.

**Clinical Pearl / High-Yield Fact**
Azotemia can be prerenal, renal, or postrenal in origin. Decreased GFR is a key factor in the development of azotemia, regardless of the underlying cause.

**Correct Answer:** D. 50%