**Core Concept**
The triple response of Lewis is a physiological response observed in the skin after stroking or rubbing it with a firm object. It consists of three distinct phases: the red response, the flare response, and the wheal response. The red response is characterized by a red line or area along the path of the stimulus, resulting from the dilation of arterioles.

**Why the Correct Answer is Right**
The red response is mediated by the release of histamine from mast cells in the skin. Histamine causes the dilation of arterioles, leading to an increase in blood flow and resulting in the red coloration of the skin. This response is an example of a local inflammatory reaction, which is a nonspecific response to tissue injury or irritation.

**Why Each Wrong Option is Incorrect**
**Option A:** This option is incorrect because it does not specify the mechanism underlying the red response.
**Option B:** This option is incorrect because it does not relate to the physiological response observed in the triple response.
**Option C:** This option is incorrect because it does not accurately describe the cause of the red response.

**Clinical Pearl / High-Yield Fact**
The triple response is an important concept in dermatology, as it helps to understand the pathophysiology of allergic reactions and other skin conditions. The release of histamine from mast cells is a key event in the triple response, and it can also play a role in other conditions such as anaphylaxis and urticaria.

**Correct Answer: C. Histamine.**