Final answer:
In an up and over down and under collision, a leg injury is the least likely mechanism of injury as the legs are not typically the primary point of impact.
Step-by-step explanation:
In the context of an "up and over" or "down and under" collision, the least likely mechanism of injury is a leg injury. This type of collision involves a rapid deceleration event where the body is thrust forward and upward (up and over) or downward and under the dashboard (down and under), typically leading to specific types of injuries. During such collisions, whiplash injuries are common due to the sudden hyperextension and flexion of the neck. Head injuries might occur as the head strikes the steering wheel or dashboard. Chest injuries are also likely because the chest can be compressed against the steering wheel or seat belt. However, leg injuries are less common in these scenarios as the legs are not the primary point of impact.