A flail chest is defined as:

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

Answer: (B)

Flail chest is defined by a portion of the chest wall becoming detached from the rest of the thoracic cage due to multiple fractures, producing a free segment that moves independently with breathing. This detached segment often moves paradoxically—sinking in during inspiration and bulging out during expiration—because it no longer shares the same mechanics as the rest of the chest wall. The key feature is that there is a detached segment of the chest wall, not just a list of fractures or a bulge in one phase of breathing. That’s why this description best captures the defining phenomenon of flail chest. The other statements describe related injuries or scenarios (bulging during inhalation, bilateral or excessive rib fractures on one side) but do not pinpoint the essential detached, paradoxically moving chest-wall segment.