What defines an open pneumothorax?

An open pneumothorax is defined as a defect in the chest wall that allows air to enter the pleural space, particularly during inhalation. This condition typically occurs due to a penetrating injury, such as a stab wound or gunshot wound, creating a direct path for air to move in and out of the pleural cavity. The presence of this defect effectively disrupts the normal pressure differential in the thoracic cavity, leading to respiratory difficulties as the lung on the affected side may not be able to expand properly.

In contrast, a complete collapse of a lung due to fluid accumulation refers to a different pathology, such as a pleural effusion or hemothorax, rather than an open pneumothorax. Similarly, a fracture of the sternum leading to internal bleeding describes a situation involving trauma but does not pertain specifically to the mechanics of air entering the pleural space. Lastly, a condition where the pleural space is filled with pus indicates an empyema, which is again distinct from the definition of an open pneumothorax. Thus, the definition provided in the first choice accurately characterizes this type of injury.