Final answer:
Flail chest is usually caused by trauma or injury. It involves a segment of the rib cage breaking and detaching from the chest wall. A fall onto an outstretched hand or a blow to the shoulder can lead to a common fracture of the clavicle or other shoulder area injuries. The correct option is B.
Step-by-step explanation:
A flail chest is usually caused by B) Trauma or injury. This serious condition occurs when a segment of the rib cage breaks due to trauma and becomes detached from the rest of the chest wall. Two possible injuries of the pectoral girdle that may occur following a strong blow to the shoulder or a hard fall onto an outstretched hand are fractures of the clavicle and dislocations or fractures of the shoulder area.
The clavicle is the most commonly fractured bone in the body. This is often due to the force exerted on it when a person falls onto an outstretched arm, causing the clavicle to break typically between the middle and lateral portions of the bone. Upon a complete fracture, the shoulder will drop, and the person might support the injured arm with their other hand. In contrast, the sternoclavicular joint is strong, resulting in the clavicle taking the brunt of the force from such impacts.
Regarding intercostal muscle contraction, the contraction of the external intercostal muscles causes the rib cage to lift and expand rather than compress, which facilitates inhalation. It does not cause the diaphragm to move downward; that is the result of diaphragm contraction.