Standing on an old stool, Lisa was cleaning the top shelf of her kitchen. Unfortunately, she slipped and fell on her knee. The knee swelled up immediately, and the pain became unbearable. When the...

Standing on an old stool, Lisa was cleaning the top shelf of her kitchen. Unfortunately, she slipped and fell on her knee. The knee swelled up immediately, and the pain became unbearable. When the doctor examined her, the knee was warm to touch as it was filled with fluid. What has happened to Lisa?