Which statement BEST defines a flail chest?

• A. A segment of the chest wall is detached from the thoracic cage.
• B. A segment of fractured ribs bulges during the inhalation phase.
• C. Multiple ribs are fractured on both sides of the chest.
• D. More than three ribs are fractured on the same side.

Answer: (A)

Flail chest is defined by a segment of the chest wall that loses its attachment to the rest of the thoracic cage because multiple adjacent ribs are fractured. That detached segment becomes unstable and moves paradoxically during breathing—moving inward with inspiration and outward with expiration, opposite to the rest of the chest.

The statement describing detachment captures this fundamental issue of chest-wall instability. The other options describe features that are not defining: bulging during inhalation misstates the direction of movement; bilateral fractures aren’t required for a flail segment to exist; and having more than three ribs fractured on one side describes extent rather than the essential detached segment.