## **Core Concept**
The Lewis triple response, also known as the triple response of Lewis, is a physiological response of the skin to mild damage, such as stroking or applying pressure. It involves three main reactions: red line (or line of demarcation), flare, and wheal. This response is primarily mediated by histamine release from damaged skin cells and the subsequent interaction with histamine receptors on blood vessels.

## **Why the Correct Answer is Right**
The Lewis triple response is primarily mediated by **histamine**, a chemical mediator released from mast cells and basophils in response to tissue injury. The response involves:
- **Red line (or line of demarcation):** Immediate localized vasoconstriction due to direct effect on blood vessels.
- **Flare:** A surrounding area of erythema (redness) caused by histamine-induced vasodilation and increased permeability of nearby blood vessels, spreading through the **axon reflex**.
- **Wheal:** A localized area of edema (swelling) resulting from increased vascular permeability due to histamine binding to H1 receptors on endothelial cells.

## **Why Each Wrong Option is Incorrect**
- **Option A:** While acetylcholine is a neurotransmitter involved in various physiological responses, it is not directly responsible for mediating the Lewis triple response.
- **Option B:** Adrenaline (epinephrine) is involved in the body's 'fight or flight' response and affects blood pressure and heart rate but is not the mediator of the Lewis triple response.
- **Option C:** This option might seem plausible due to the involvement of the nervous system in the flare response, but the direct mediator of the Lewis triple response is not a neurotransmitter like substance P but rather histamine.

## **Clinical Pearl / High-Yield Fact**
The Lewis triple response is often used as a clinical test to assess the integrity of the **dermal circulation** and **neurovascular reflexes**. A diminished or absent response may indicate conditions such as peripheral neuropathy or severe vascular disease.

## **Correct Answer:** . **histamine**