Which external forces can cause a head injury?

A head injury can result from various external forces, and the most relevant factors typically involve direct physical impacts or shocks. A blow to the head, being within 50 meters of a blast, or being involved in a vehicle accident represents significant trauma that can lead to head injury. Each of these scenarios involves an external force that directly affects the head or the skull, potentially causing concussions, contusions, or more severe traumatic brain injuries.

Excessive sunlight exposure does not cause head injuries as it pertains more to skin damage or heat-related illnesses rather than physical trauma to the head. Inadequate hydration affects overall bodily function and can lead to other health issues, but it does not directly result in head injuries. Similarly, improper lifting techniques may cause back or joint injuries, but they are unlikely to produce trauma to the head unless the lifting technique somehow results in a fall or a related accident. Therefore, the option that encompasses direct external forces related to head trauma is indeed the correct one.