<prompt> If you are a doctor, please answer the medical questions based on the patients description.Doctor, I have been experiencing joint stiffness and tightness, and I have a lump on my knee. Can you tell me what might be the cause of these symptoms?  <response>: Based on your symptoms, it is possible that you have a cytomegalovirus infection. It is a common viral infection that can affect various parts of the body, including the joints, leading to stiffness and swelling. The lump on your knee could also be a result of this infection. 