How is a longitudinal fracture primarily caused?

A longitudinal fracture occurs when stress is applied along the length of the bone, leading to a fracture that runs parallel to the bone's long axis. This type of fracture typically arises from forces that act to stretch or pull the bone in opposite directions along its length, resulting in a crack that aligns with the bone’s structure.

This distinguishes it from other fracture types which may be caused by different forces. For instance, compressive forces can lead to other fracture patterns such as transverse or oblique fractures rather than a longitudinal one. Similarly, a sudden impact from the side tends to cause more lateral or transverse fractures, while bending the bone outward would typically result in bending fractures or other forms of fractures that do not align with the bone's long axis. Therefore, the correct option centers around the mechanism of stress aligned with the bone's direction.