MSRA-1395

A 26-year-old construction worker visits his GP complaining of elbow swelling that has been present for three weeks. He reports that it started gradually and has no known triggers. The area is painful and warm to the touch, but he has no swelling in other parts of his body and feels generally well. The patient has a history of well-managed rheumatoid arthritis and is taking methotrexate, and has no other medical conditions. During the examination, the doctor detects a tender, soft, fluctuant mass on the back of the patient’s elbow. Based on these findings, what is the most probable diagnosis?