Which of the following describes a pneumothorax?

A pneumothorax occurs when air leaks into the pleural space, the area between the lungs and the chest wall. This situation can lead to a collapse of the lung on the affected side, causing difficulty in breathing and chest pain. The presence of air in the pleural space disrupts the normal negative pressure that is necessary for proper lung function, potentially leading to respiratory distress.

Understanding this definition helps clarify why the other descriptions, while related to respiratory conditions, do not align with what a pneumothorax specifically entails. For instance, gas trapped in lung tissue relates to other pulmonary issues, fluid accumulation in the pleural space describes pleural effusion, and infection in the alveolar region refers to pneumonia. These conditions are distinct from a pneumothorax and involve different physiological processes and implications for treatment.