mrcem2-1070

A 12-year-old girl presents with her father. She is very active and enjoys participating in various sports at a high level. Lately, she has been worried about experiencing pain in her right ankle. There is no history of injury, instability, or swelling. She is in good overall health. She can walk and run without discomfort, and the pain does not worsen after prolonged periods of rest. During the examination of her ankle, you observe tenderness and slight swelling around the lateral malleolus.

What is the SINGLE most likely diagnosis?