**Core Concept**
The question is testing the understanding of the mechanism of glomerular filtration and the role of the glomerular filtration barrier in the kidney. The glomerular filtration barrier allows the selective passage of substances from the blood into the urine, while restricting the passage of larger molecules and cells.

**Why the Correct Answer is Right**
The presence of a substance in the afferent arteriole but not in the efferent arteriole suggests that it is being filtered out by the glomerulus. The glomerular filtration barrier, composed of the fenestrated endothelium, the glomerular basement membrane, and the podocytes, selectively allows the passage of small molecules, such as ions and water, while restricting the passage of larger molecules, such as proteins and blood cells. The substance in question is likely a small molecule that is being filtered out by the glomerulus.

**Why Each Wrong Option is Incorrect**
**Option A:** This option is incorrect because it does not provide any information about the substance or its behavior in the kidney.
**Option B:** This option is incorrect because it does not explain why the substance is being filtered out by the glomerulus.
**Option C:** This option is incorrect because it does not provide a clear explanation for the phenomenon observed.

**Clinical Pearl / High-Yield Fact**
It is essential to remember that the glomerular filtration barrier is highly selective, allowing the passage of small molecules while restricting larger molecules. This is crucial in maintaining the balance of electrolytes and preventing the loss of essential proteins in the urine.

**Correct Answer: C. The substance is a small molecule that is being filtered out by the glomerulus.**