What is a common sign of an ACL injury?

A common sign of an ACL injury is the experience of a popping sound during movement, often reported by athletes and individuals involved in high-impact sports. This popping sensation typically occurs at the moment of injury when the ligament is torn, and it's frequently accompanied by immediate pain. The knee may feel unstable, and individuals may struggle to continue any physical activity after this sound is heard, as it indicates a significant injury to the ligament that is crucial for knee stability and function.

In the context of the other options, while swelling around the ankle, localized hip pain, and bruising under the foot can indicate various injuries or conditions, they are not specific to an ACL injury. Swelling around the ankle might suggest an ankle sprain or other lower leg injuries, localized hip pain could be associated with hip joint issues, and bruising under the foot may result from impacts or contusions but does not relate to the knee or ACL damage. Thus, the unique association of the popping noise with ACL injuries distinguishes it as the primary sign indicative of such an injury.