**Question:** Number of cilia in bronchiole

**Core Concept:** Bronchioles are small airways that branch from the bronchi and further divide the lungs into a network of terminal bronchioles. Cilia are hair-like projections found on the surface of respiratory epithelial cells that play a crucial role in respiratory defense mechanisms, including mucociliary clearance and deflection of inhaled particles and pathogens.

**Why the Correct Answer is Right:** The correct answer, **C. 1,000-2,000**, represents the approximate number of cilia present in the respiratory tract, including bronchiole. Each bronchiole is lined by pseudostratified columnar ciliated epithelium, which is crucial for maintaining respiratory health by clearing inhaled particles and pathogens from the airways.

**Why Each Wrong Option is Incorrect:**

A. **10-20 cilia**: This option is incorrect because the number of cilia varies among different airways and is not a fixed value.

B. **None**: This option is incorrect as cilia are present on the respiratory epithelium and serve vital functions in respiratory defense mechanisms.

D. **Varied**: This option is incorrect, as the number of cilia in bronchiole is not variable and ranges from 1,000-2,000.

**Clinical Pearl:** Understanding the number and function of cilia in bronchiole is essential for appreciating the role of mucociliary clearance in respiratory health and disease management. In diseases like cystic fibrosis, the impaired function of cilia can lead to impaired mucociliary clearance, resulting in lung infections and inflammation.