Which of the following conditions is least likely to cause joint pain after a fall?

Joint pain following a fall is often linked to conditions directly related to injury or inflammation in the joints. A fracture is a common result of a fall and can certainly lead to joint pain, as the trauma directly impacts the area. Reactive arthritis occurs as a response to an infection elsewhere in the body and can lead to joint pain. Septic arthritis is an infection within the joint fluid and tissues, typically arising after injury or due to a bacterial invasion, which can also cause significant joint pain.

Mononucleosis, typically caused by the Epstein-Barr virus, primarily presents with symptoms like fatigue, fever, and sore throat. While it can lead to mild joint pain, especially from general malaise, it is generally not associated with direct trauma or injury from a fall. Therefore, among the options provided, mononucleosis is least likely to cause joint pain specifically after a fall, as it does not have a direct connection to joint trauma or injury like the other conditions do.