Pneumothorax means air in the pleural cavity, collapsed lung.

• A. Infection of the pleural space
• B. Fluid in the pleural space
• C. Air in the pleural cavity, collapsed lung
• D. Puncture of the chest wall

Answer: (C)

Pneumothorax is defined by air in the pleural space, which disrupts the negative pressure that normally keeps the lung expanded, causing the lung to collapse. The pleural space is a potential space between the lung and chest wall; when air enters it, the balance of pressures changes and the lung cannot stay inflated, leading to partial or complete collapse. This exact description—air in the pleural cavity with a collapsed lung—is what pneumothorax means. The other options describe different pleural conditions or mechanisms: infection of the pleural space would be an empyema, fluid in the pleural space is a pleural effusion, and a puncture of the chest wall is a possible cause but not the definition itself.