Cystic fibrosis primarily results in which physiologic problem?

• A. Thick mucus that impairs oxygen exchange
• B. Excess mucus production improving clearance
• C. Weak cough with silenced airway reflex
• D. Decreased mucus viscosity

Answer: (A)

Cystic fibrosis primarily causes thick, sticky mucus in the airways that obstructs small airways and impairs oxygen exchange. The defective chloride transport leads to dehydrated, highly viscous secretions, so mucus plugs reduce ventilation to parts of the lung, causing ventilation–perfusion mismatch and hypoxemia. This thick mucus also promotes recurrent infections and inflammation, further compromising gas exchange.

Mucus that is thick, not thin, does not help clearance; a weak or silenced cough isn’t typical, since cough is usually present but often ineffective due to the viscosity of the secretions; and decreased mucus viscosity would not describe CF, which features increased mucus viscosity.