**Core Concept**
Uremia is a condition characterized by the accumulation of waste products in the blood, typically resulting from impaired renal function. The glomerular filtration rate (GFR) is a critical measure of kidney function, and a reduction in GFR can lead to the retention of waste products, including urea.

**Why the Correct Answer is Right**
The correct answer is based on the concept that uremia occurs when the kidneys are unable to effectively remove waste products from the blood. The GFR is a measure of the rate at which the kidneys filter waste products from the blood. A reduction in GFR indicates that the kidneys are not functioning properly, leading to the accumulation of waste products in the blood. Specifically, a total GFR of less than 10% of normal is typically considered to be a threshold for severe renal impairment, at which point uremia can occur.

**Why Each Wrong Option is Incorrect**
**Option A:** This option is incorrect because a reduction in GFR alone is necessary but not sufficient to cause uremia. Other factors, such as changes in blood flow or the presence of nephrotoxins, can also contribute to the development of uremia.

**Option B:** This option is incorrect because a reduction in GFR does not necessarily cause uremia. The kidneys have a remarkable capacity to compensate for reductions in GFR, and uremia typically occurs when GFR is severely impaired.

**Option C:** This option is incorrect because a reduction in GFR is a necessary condition for uremia, but it is not a sufficient condition. Other factors, such as changes in blood flow or the presence of nephrotoxins, can also contribute to the development of uremia.

**Clinical Pearl / High-Yield Fact**
It's essential to remember that uremia can occur even when serum creatinine levels are within the normal range, especially in patients with chronic kidney disease. This highlights the importance of monitoring GFR in patients with kidney disease.

**Correct Answer: C. 15% of normal GFR**