A pneumothorax is:

• A. Fluid accumulation in the pleural space
• B. The introduction of air into the pleural cavity
• C. Inflammation of the pleura
• D. Collapse of the alveoli

Answer: (B)

A pneumothorax is air in the pleural space, the thin gap between the lung and chest wall. This air disrupts the negative pressure that helps keep the lung expanded, so the lung on the affected side can partially or fully collapse. That direct description—air entering the pleural cavity—is what defines a pneumothorax.

The other descriptions refer to different problems: fluid in the pleural space is a pleural effusion; inflammation of the pleura is pleuritis or pleurisy; collapse of the alveoli is atelectasis. Pneumothorax can lead to breathing difficulties and diminished breath sounds, but the defining feature is the presence of air in the pleural space.