## Core Concept
The question tests the understanding of renal handling of various substances, specifically focusing on the processes of filtration, secretion, and reabsorption in the renal tubules. The renal tubules play a crucial role in maintaining electrolyte balance, regulating pH, and eliminating waste products through these mechanisms.

## Why the Correct Answer is Right
Inulin is a polysaccharide that is often used as a marker for glomerular filtration rate (GFR) because it is primarily filtered at the glomerulus and neither secreted nor reabsorbed by the renal tubules. This characteristic makes inulin an ideal substance for measuring the filtration capacity of the kidneys. The renal clearance of inulin is equal to the GFR, which is approximately 120 mL/min in a healthy adult.

## Why Each Wrong Option is Incorrect
* **Option A:** Glucose is primarily reabsorbed in the proximal convoluted tubule (PCT) back into the bloodstream. Under normal conditions, almost all filtered glucose is reabsorbed, and its clearance is close to zero, making it an incorrect choice.
* **Option B:** Creatinine, a product of muscle metabolism, is both filtered and secreted by the renal tubules. Its clearance is often used as an estimate of GFR but is not as accurate as inulin clearance because of the additional secretory component.
* **Option D:** Urea is handled by the kidneys in a complex manner; it is filtered and then partially reabsorbed back into the bloodstream, especially in states of low urine flow. This reabsorption can vary depending on hydration status and other factors.

## Clinical Pearl / High-Yield Fact
A key point to remember is that inulin clearance is the gold standard for measuring GFR. However, due to the complexity of inulin clearance measurement, creatinine clearance is commonly used in clinical practice as a practical estimate of GFR, despite its limitations.

## Correct Answer Line
**Correct Answer: C. Inulin.**