**Core Concept:** The substance in question is likely a neurotransmitter or a drug that diffuses across the synaptic cleft during neurotransmission or cellular uptake. In the afferent arteriole, the substance has a concentration of 2 mg%, which means it is present at a concentration of 2 mg% in the afferent arteriole and zero mg% in the efferent arteriole.

**Why the Correct Answer is Right:** The correct answer is **D** because it refers to a substance that is present at a concentration of 2 mg% in the afferent arteriole, which is the side of the arteriole where the substance is released from the surrounding cells and diffuses across the synaptic cleft to bind to receptors on the smooth muscle cells. Since the substance is not present in the efferent arteriole, where it is removed by cellular uptake or reabsorption, we can conclude that the substance is actively transported from the afferent arteriole to the efferent arteriole.

**Why Each Wrong Option is Incorrect:**

A. This option is incorrect because it suggests that the substance is uniformly distributed throughout the capillary bed, which is not the case as it is specifically present in the afferent arteriole and absent in the efferent arteriole.

B. This answer is wrong because it implies that the substance is present at a concentration of 2 mg% in the efferent arteriole, which contradicts the information provided in the question.

C. This answer is incorrect because it suggests that the substance is uniformly distributed within the arteriole, which contradicts the information provided that it is present at a concentration of 2 mg% in the afferent arteriole and zero mg% in the efferent arteriole.

**Clinical Pearl:** Understanding the distribution of substances between arterioles is essential for understanding the physiological processes in the kidney, particularly in the regulation of blood pressure and fluid and electrolyte homeostasis. This concept is important in understanding how the kidneys filter blood and regulate blood pressure, as well as in determining the efficacy of drugs targeting the renal vasculature.