**Core Concept**
The filtration fraction is the ratio of the glomerular filtration rate (GFR) to the renal plasma flow (RPF). It is an important parameter in assessing the kidney's ability to filter waste products from the blood. The filtration fraction is influenced by the glomerular capillary pressure, the surface area of the glomeruli, and the permeability of the glomerular basement membrane.

**Why the Correct Answer is Right**
The correct answer is **Inulin clearance**. Inulin is a polysaccharide that is neither reabsorbed nor secreted by the renal tubules, making it an ideal substance for measuring GFR. By measuring the clearance of inulin, we can accurately determine the GFR, which can then be used to calculate the filtration fraction. The filtration fraction is calculated as the ratio of inulin clearance to para-amino hippurate (PAH) clearance, which is a marker of RPF.

**Why Each Wrong Option is Incorrect**
**Option B:** Creatinine clearance is not a reliable measure of GFR because creatinine is both reabsorbed and secreted by the renal tubules, which can lead to overestimation of GFR.
**Option C:** Urea clearance is not a good measure of GFR because urea is reabsorbed by the renal tubules, especially in conditions of low urine flow.
**Option D:** The fractional excretion of sodium (FeNa) is a measure of the amount of sodium excreted in the urine relative to the amount filtered by the glomeruli, but it does not provide a direct measure of GFR.

**Clinical Pearl / High-Yield Fact**
Remember that inulin clearance is the gold standard for measuring GFR, and it is essential to use a substance that is neither reabsorbed nor secreted by the renal tubules to accurately determine GFR.

**Correct Answer:** A. Inulin clearance