Step 4: Read and complete the decision guide to learn more about your symptoms.
Based on your answers (you are older than 45, no other joints are bothering you, your painful knee is swollen and there is no locking and/or giving way of the knee), there are a number of possible causes of knee pain. More than one may be present at the same time, including:
crystal-induced disease such as gout and pseudogout
infection (less likely)
ligament or cartilage damage (though one of these would be even more likely if you had locking or giving-way)
other type of arthritis called spondyloarthropathy.