**Question:** Triple response is due to -

A. Mucus secretion
B. Bronchoconstriction
C. Increased vascular permeability
D. Alveolar edema

**Core Concept:** The triple response is a series of three immediate responses that occur in the respiratory system following exposure to an antigen or irritant. These responses are:

1. **Mucus secretion:** The first response involves the production and secretion of mucus, which helps trap and remove the allergen or irritant from the airways.
2. **Bronchoconstriction:** The second response involves the contraction of airway smooth muscle, leading to narrowing of the airways and increased resistance to airflow.
3. **Increased vascular permeability:** The third response involves an increase in the permeability of blood vessels, causing fluid to leak into the airways and surrounding tissues, which leads to airway edema and further narrowing of the airways.

**Why the Correct Answer is Right:** The correct answer, C. Increased vascular permeability, is the result of the release of histamine and other inflammatory mediators during the immune response to an allergen or irritant. These mediators cause the endothelium of blood vessels to become more permeable, allowing plasma proteins to enter the interstitial space and cause airway edema. This process contributes to the overall inflammation and narrowing of the airways, leading to the triple response.

**Why Each Wrong Option is Incorrect:**

A. Mucus secretion (Option A) is the first response in the triple response and is not a result of increased vascular permeability.

B. Bronchoconstriction (Option B) is also a component of the triple response, but it is caused by the contraction of airway smooth muscle, not increased vascular permeability.

D. Alveolar edema (Option D) is a separate process called the "alveolar exudate response," which occurs after the initial triple response and involves the accumulation of fluid in the alveoli, leading to pulmonary edema. It is not directly related to the triple response.

**Clinical Pearl:** The triple response is a crucial aspect of the immune response to allergens or irritants and is essential for the body to clear the offending agent from the airways. It is essential for medical students to understand the sequence and mechanisms of the triple response, as it is a fundamental concept in respiratory medicine and can affect the clinical management of patients with respiratory symptoms.