A 13-year-old girl and her father have come to emergency department for assessment of the girl's stiff and warm knee. The father reports that she hurt it sliding into base during a softball game, but later in the interview it becomes clear that the slide made an existing pain worse. Her knee had been stiff, warm, and shiny, and had been keeping her awake at night for a week or so before the game. Which condition might be the cause of the girl's symptoms