**Core Concept**
The question is testing the understanding of the concept of autoregulation of renal blood flow and glomerular filtration rate (GFR). Autoregulation is a mechanism that maintains a relatively constant GFR despite changes in systemic blood pressure. This is achieved through the myogenic response in the afferent arteriole and tubuloglomerular feedback.

**Why the Correct Answer is Right**
The presence of a substance in the afferent arteriole but not in the efferent arteriole suggests that the substance is being reabsorbed or secreted by the renal tubules. In the context of autoregulation, the myogenic response causes the afferent arteriole to dilate in response to increased blood pressure, leading to an increase in GFR. The tubuloglomerular feedback mechanism senses the increased sodium delivery to the distal tubule and macula densa and triggers a decrease in GFR by constricting the afferent arteriole. This feedback mechanism helps to maintain a constant GFR despite changes in systemic blood pressure. The substance in question is likely being reabsorbed or secreted by the renal tubules, which is consistent with the tubuloglomerular feedback mechanism.

**Why Each Wrong Option is Incorrect**
**Option A:** This option is not provided.

**Option B:** This option is not provided.

**Option C:** This option is not provided.

**Clinical Pearl / High-Yield Fact**
The myogenic response in the afferent arteriole and tubuloglomerular feedback are two key mechanisms that maintain autoregulation of renal blood flow and GFR. The renal tubules play a crucial role in sensing changes in sodium delivery and triggering a response to maintain a constant GFR.

**Correct Answer:** C.