Which sign is characteristic of a flail chest?

• A. Cyanosis only with exertion
• B. Unequal chest expansion
• C. Paradoxical chest movement
• D. Pleural friction rub

Answer: (C)

Paradoxical chest movement is the defining sign of a flail chest. When multiple adjacent ribs are fractured in several places, a detached segment of the chest wall moves independently from the rest of the chest. During inspiration, the intact chest expands but the free segment moves inward, and during expiration it moves outward. This opposite, involuntary motion—the paradoxical movement—reflects chest-wall instability and severe underlying trauma. The other signs can occur with different conditions: cyanosis that appears only with exertion points to generalized hypoxemia rather than chest-wall instability; unequal chest expansion can result from pneumothorax, atelectasis, or other problems; pleural friction rub indicates pleural inflammation, not a flail segment.