**Question:** Onion skin thickening of airway wall is seen in-

A. Asthma
B. Chronic bronchitis
C. Chronic obstructive pulmonary disease (COPD)
D. Allergic rhinitis

**Core Concept:**
Onion skin thickening refers to the pathological process where the airway wall becomes progressively thickened, a condition known as airway remodeling. This occurs due to chronic inflammation and tissue remodeling processes in response to various respiratory diseases.

**Why the Correct Answer is Right:**
The correct answer, **C. Chronic obstructive pulmonary disease (COPD)**, is related to onion skin thickening because COPD is characterized by long-term inflammation and airway remodeling. In COPD, the airway walls undergo progressive thickening due to increased muscle mass, collagen deposition, and goblet cell hyperplasia. This leads to airway obstruction and worsening respiratory symptoms.

**Why Each Wrong Option is Incorrect:**

A. **Asthma** is a condition characterized by reversible airway narrowing, not thickening. Although asthma can cause inflammation and remodeling, the airway wall does not undergo the thickening seen in onion skin pattern.

B. **Chronic bronchitis** is a condition characterized by chronic inflammation of the airways, but not specifically onion skin thickening. Chronic bronchitis does not lead to the same specific pathological changes seen in onion skin thickening.

D. **Allergic rhinitis** is an allergic condition affecting the nasal passages, not the airway walls. Onion skin thickening is specific to airway inflammation and remodeling, not observed in allergic rhinitis.

**Clinical Pearl:**
Awareness of onion skin thickening helps differentiate between various respiratory diseases. The presence of onion skin thickening in airway biopsy samples can help clinicians diagnose and tailor appropriate treatment plans for patients with COPD or asthma, as the therapeutic strategies differ between these conditions. In COPD, bronchodilators and corticosteroids are typically prescribed to reduce inflammation and improve airway caliber. In asthma, bronchodilators are essential, but corticosteroids are crucial for reducing inflammation and preventing asthma exacerbations.