Three or more ribs fractured in two or more places defines which injury?

The correct identification of the injury described in the question is due to the specific definition of flail chest. A flail chest occurs when a segment of the rib cage becomes detached from the rest of the thoracic wall, which typically happens when three or more adjacent ribs are fractured at two or more sites. This creates a paradoxical movement of the chest wall; during inhalation, the affected segment moves inward instead of expanding, leading to inadequate ventilation and potentially compromising oxygenation.

This injury is critical to recognize and differentiate from other types of rib injuries. Pneumothorax, for example, involves air accumulation in the pleural space and is not characterized by rib fractures or the unique movement associated with flail chest. A rib contusion refers to a bruise on the rib without fractures, and a simple rib fracture describes a single fracture of the rib without fragmentation or multiple sites of fracture. Therefore, the definition aligns specifically with the condition of flail chest, making it the correct answer in this context.