**Core Concept**
Uremia is a clinical syndrome characterized by the accumulation of waste products, primarily urea, in the blood due to impaired renal function. The glomerular filtration rate (GFR) is a critical parameter in assessing kidney function, and a reduction in GFR leads to the accumulation of these waste products, resulting in uremia.

**Why the Correct Answer is Right**
The GFR is the rate at which the kidneys filter waste products from the blood. A significant reduction in GFR, typically below 30-40 mL/min/1.73 m^2, leads to the accumulation of urea and other waste products in the blood, resulting in uremia. This is because the kidneys are unable to effectively filter and excrete these waste products, leading to their buildup in the body.

**Why Each Wrong Option is Incorrect**
* **Option A:** A reduction in GFR is necessary for uremia to occur, but it is not the only factor. Other conditions, such as obstruction or inflammation, can also lead to uremia.
* **Option B:** A GFR of 60 mL/min/1.73 m^2 is within the normal range and would not lead to uremia.
* **Option C:** A GFR of 30-40 mL/min/1.73 m^2 is typically considered to be moderately impaired, but it is not the threshold for uremia.

**Clinical Pearl / High-Yield Fact**
Uremia is a clinical syndrome that can be caused by a variety of factors, including acute kidney injury, chronic kidney disease, and kidney obstruction. It is essential to assess GFR and other renal function parameters to diagnose and manage patients with uremia.

**Correct Answer: C**. 30-40 mL/min/1.73 m^2