<prompt> If you are a doctor, please answer the medical questions based on the patients description.Doctor, I fell down and injured my leg. Since then, my knee has been swelling. What could be wrong?  <response>: Based on the symptoms you have described, you may be suffering from knee effusion, which is a buildup of excess fluid in or around your knee joint due to injury or inflammation. I will need to conduct an examination to confirm the diagnosis. 