Final answer:
The nurse should identify the client's chest movement inward during inspiration and bulging outward during expiration as flail chest, a serious condition often resulting from trauma.
Step-by-step explanation:
The nurse observing a client's chest movement inward during inspiration and bulging outward during expiration should identify this finding as flail chest. Flail chest is typically caused by a trauma such as a motor-vehicle crash and manifests as a segment of the rib cage breaking and becoming detached from the rest of the chest wall. This condition leads to this characteristic movement because the detached part of the chest moves in the opposite direction to the rest of the chest wall during breathing. It's crucial in an emergency setting to recognize this pattern of chest movement as it requires specific medical intervention.