Mucus excess and poor clearance trigger COPD cough

Steps:

• Cough reflex clears excess mucus and irritants from airways in COPD.
• Change 4 boosts goblet cell mucus secretion, irritating airways and stimulating chronic cough.
• Change 3 inactivates cilia, blocking mucus transport and causing retention that provokes cough.
• Changes 1 and 2 impair gas exchange (emphysema) and promote infections, linking to breathlessness and acute episodes, not chronic cough.

Why D is correct:

• COPD cough stems from chronic bronchitis, defined by hyperplasia of mucus glands (change 4) and ciliary paralysis (change 3), per WHO classification.

Why the others are wrong:

• A includes 1 (alveolar collapse causes dyspnea) and 2 (bacteria cause infections, not routine cough).
• B pairs 1 (emphysema-related, not cough) with 3 (valid but incomplete).
• C includes 2 (infection trigger, not chronic mucus issue) with 3.

Final answer: D