A 58-year-old male smoker presents with chronic cough and hemoptysis. Bronchoscopy reveals columnar epithelium with patches of stratified squamous epithelium in the bronchi. Which is the most common site of metaplasia in the respiratory tract?

Explanation

## Metaplasia in the Respiratory Tract ### Definition and Mechanism Metaplasia is the reversible replacement of one differentiated cell type with another. In the respiratory tract, chronic irritation (smoking, pollution, chronic inflammation) triggers squamous metaplasia of the normal ciliated columnar epithelium. ### Most Common Site: Bronchi and Bronchioles **Key Point:** The bronchi and bronchioles are the most frequent sites of squamous metaplasia in the respiratory tract because they are directly exposed to inhaled irritants and carcinogens. **High-Yield:** Squamous metaplasia in the bronchi is a classic precancerous lesion and represents an adaptive response to chronic injury. The sequence is: 1. Chronic irritation (smoking, pollution) 2. Loss of ciliated columnar epithelium 3. Replacement with stratified squamous epithelium 4. Potential progression to dysplasia and carcinoma ### Comparative Sites | Site | Frequency of Metaplasia | Mechanism | Clinical Significance | |------|-------------------------|-----------|----------------------| | **Bronchi/Bronchioles** | Most common | Direct exposure to irritants | Precancerous; strong SCC risk | | Trachea | Less common | Protected by larynx; less direct exposure | Rare metaplasia | | Larynx | Uncommon | Upper airway; less chronic irritation | Occasional in heavy smokers | | Alveoli | Never | Respiratory epithelium; different physiology | No metaplasia occurs | **Clinical Pearl:** Squamous metaplasia of bronchi is reversible if the irritant is removed early (e.g., smoking cessation), but prolonged exposure leads to dysplasia and eventual squamous cell carcinoma of the lung. **Mnemonic:** **BABE** — Bronchi and Bronchioles Are the Best sites for squamous metaplasia in the respiratory tract.