What are the two main indications for X-ray of the ankle?

The primary indications for performing an X-ray of the ankle are indeed related to the mechanism of injury and specific clinical findings that suggest a fracture may be present. Among these indicators, the inability to bear weight for at least four steps after the injury and tenderness over the posterior malleolus are key components of the Ottawa Ankle Rules.

The rationale behind these specific indications is to ensure that only patients with a significant likelihood of fracture receive X-ray imaging, thereby reducing unnecessary radiation exposure and healthcare costs. The inability to walk reflects the extent of the injury and its impact on function, while tenderness over the posterior malleolus specifically indicates potential injury to the ankle's bony structures.

Other options may mention conditions like bone swelling or instability, which could suggest an injury needing further evaluation, but they do not align specifically with the established clinical guidelines for X-ray determination like the Ottawa Ankle Rules do. Thus, the first choice encapsulates the most direct criteria for deciding to obtain an X-ray of the ankle.