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?

Question

Accepted Answer

B. Bronchi and bronchioles. ## 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.

Answer

A. Larynx

Answer

C. Alveoli

Answer

D. Trachea

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

Most Common Site: Bronchi and Bronchioles

Comparative Sites

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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