**Core Concept**
Uremia is a clinical syndrome associated with **chronic kidney disease** and **acute kidney injury**, characterized by the accumulation of waste products in the blood. The **glomerular filtration rate (GFR)** is a key indicator of kidney function.

**Why the Correct Answer is Right**
A reduction in GFR leads to decreased excretion of waste products, such as **urea**, **creatinine**, and other **nitrogenous wastes**. When the total GFR is reduced by 50% or more, the kidneys are unable to adequately filter and remove these waste products, resulting in their accumulation in the blood and leading to uremia.

**Why Each Wrong Option is Incorrect**
**Option A:** A 10% reduction in GFR is not sufficient to cause significant accumulation of waste products.
**Option B:** A 25% reduction in GFR may lead to mild impairment of kidney function but is not typically associated with uremia.
**Option D:** A 75% reduction in GFR would certainly lead to uremia, but the question asks for the threshold at which uremia occurs.

**Clinical Pearl / High-Yield Fact**
Uremia can lead to various clinical manifestations, including **azotemia**, **fluid overload**, and **electrolyte imbalances**.

**Correct Answer:** C. 50%