## **Core Concept**
Allergic rhinitis, also known as hay fever, is an allergic reaction that occurs when the immune system overreacts to particles in the air that are typically harmless, such as pollen. This reaction involves the release of various chemical mediators, including histamine, which is stored in mast cells. The key to understanding allergic rhinitis lies in the role of **preformed mediators**, such as histamine, that are released from mast cells upon allergen exposure.

## **Why the Correct Answer is Right**
The correct answer, **Histamine**, is a preformed toxin involved in the mechanism of allergic rhinitis. Histamine is stored in the granules of mast cells and basophils. When an allergen binds to IgE antibodies on the surface of these cells, it triggers the release of histamine and other mediators. Histamine then acts on various receptors in the nasal mucosa, leading to symptoms of allergic rhinitis, such as sneezing, runny nose, and nasal congestion. Histamine's role is well-established in the pathophysiology of allergic reactions, including allergic rhinitis.

## **Why Each Wrong Option is Incorrect**
- **Option A:** While involved in allergic reactions, **Leukotrienes** are not preformed; they are synthesized de novo after cell activation.
- **Option B:** **Prostaglandins** are also synthesized de novo and play a role in inflammation but are not the primary preformed toxin in allergic rhinitis.
- **Option D:** **Bradykinin** is involved in the inflammatory response and can contribute to symptoms of allergic rhinitis, but it is not a preformed mediator stored in mast cells.

## **Clinical Pearl / High-Yield Fact**
A key point to remember is that **histamine** is the primary preformed mediator responsible for the immediate symptoms of allergic rhinitis. Antihistamines, which block histamine receptors, are a common and effective treatment for alleviating these symptoms.

## **Correct Answer:** . Histamine