**Core Concept**
The Glomerular Filtration Rate (GFR) is a crucial indicator of kidney function, reflecting the rate at which the kidneys filter waste products from the blood. To accurately measure GFR, the chosen substance must be freely filtered by the glomeruli, neither secreted nor reabsorbed by the renal tubules, and not metabolized by the kidneys.

**Why the Correct Answer is Right**
An ideal substance for measuring GFR should have the following characteristics:
- It should be freely filtered by the glomeruli, allowing it to pass through the glomerular basement membrane and enter the renal tubules.
- It should not be reabsorbed or secreted by the renal tubules, ensuring that the amount of substance excreted in the urine accurately reflects its filtration rate.
- It should not be metabolized by the kidneys, as this would alter its concentration in the urine and lead to inaccurate GFR measurements.

**Why Each Wrong Option is Incorrect**
* **Option A:** is not an ideal substance for measuring GFR as it is not freely filtered by the glomeruli.
* **Option B:** is not an ideal substance for measuring GFR as it is actively secreted by the renal tubules, leading to an overestimation of GFR.
* **Option C:** is not an ideal substance for measuring GFR as it is neither freely filtered by the glomeruli nor secreted by the renal tubules, but it is also not the best option among the incorrect ones.

**Clinical Pearl / High-Yield Fact**
In clinical practice, inulin is often used as an ideal substance for measuring GFR due to its freely filtered and non-reabsorbed nature, but its use is limited by its lack of availability and the complexity of the measurement process. Inulin clearance is considered the gold standard for measuring GFR.

**Correct Answer: C.**