**Core Concept**
Acute kidney injury (AKI) is a sudden decline in kidney function, often manifesting as a reduction in glomerular filtration rate (GFR). The RIFLE (Risk, Injury, Failure, Loss of kidney function, and End-stage kidney disease) criteria are widely used to stage AKI, facilitating timely interventions and predicting outcomes.

**Why the Correct Answer is Right**
The RIFLE criteria classify AKI based on the increase in serum creatinine (sCr) levels, the degree of reduction in GFR, and the need for renal replacement therapy (RRT). To be staged as AKI, patients must meet one of the following criteria:
- An absolute increase in sCr by 0.3 mg/dL (26.5 μmol/L) or more,
- A percentage increase in sCr by 50% or more from baseline,
- A reduction in GFR by 25% or more, or
- The need for RRT.

**Why Each Wrong Option is Incorrect**
**Option A:** This option is incorrect as it does not accurately describe the RIFLE criteria for AKI staging. The criteria do not solely rely on a percentage increase in serum creatinine.
**Option B:** This option is incorrect as it is not a valid criterion for AKI staging according to the RIFLE classification.
**Option C:** This option is incorrect as it is not a specific criterion for AKI staging and may apply to other kidney-related conditions.

**Clinical Pearl / High-Yield Fact**
Remember, the RIFLE criteria are essential for timely identification and management of AKI, which can significantly impact patient outcomes. Early recognition and intervention are crucial to prevent long-term kidney damage and associated complications.

**Correct Answer: C. Reduction in GFR by 25% or more.**