COPD patients undergo metaplastic changes in lung epithelium. Chronic irritation from smoking transforms the normal respiratory epithelium into:

Correct answer: D. Stratified squamous non-keratinized epithelium (squamous metaplasia)

• A. Simple columnar epithelium is found in the intestines and gall bladder — not the product of respiratory metaplasia.
• B. Simple cuboidal epithelium lines kidney tubules and thyroid follicles — not the product of metaplasia here.
• C. Transitional epithelium (urothelium) lines the bladder and ureters — not respiratory.
• D. Correct. SQUAMOUS METAPLASIA: normal pseudostratified ciliated columnar respiratory epithelium is replaced by STRATIFIED SQUAMOUS NON-KERATINIZED EPITHELIUM in response to chronic irritation (smoking, COPD). This is a protective adaptive response, but the squamous cells have lost their ciliary function (reduced mucociliary clearance → increased infections) and can progress to dysplasia and carcinoma (squamous cell carcinoma of the lung).