Sudden knee swelling and pain may have the following causes: First, gout, when gout involves the knee joint, there can be swelling and pain on the front side of the knee joint, or inflammation of the synovial membrane in the knee joint cavity, resulting in swelling and pain in the joint, then early blood tests and x-ray examination of the knee joint should be performed for early diagnosis and treatment. Second, infection of the knee joint, when the knee joint has bacterial infection or tuberculosis infection, there can be a sudden swelling and pain of the knee, which can be accompanied by redness of the skin, then blood tests for pathogenic bacteria and MRI of the knee joint should be performed for diagnosis. Third, synovitis of the knee joint, when the knee joint is inflamed due to synovial membrane or synovial membrane tumor that causes fluid accumulation, there can be sudden knee pain and swelling, then the knee MRI should be performed for early diagnosis and treatment.