MSFinals-4467

A 7-year-old girl is brought to the Emergency Department (ED) by her parents due to a painful knee. The knee is swollen, red, hot, and tender. The patient experiences extreme pain when the joint is moved, and resists any attempts to flex it passively. Upon examination, you notice a cut on the affected knee, which the parents explain was caused by a fall on the playground. The patient has a mild fever but is otherwise healthy, without other areas of pain or additional joint involvement.
What is the most probable diagnosis?