Step 4: Read and complete the decision guide to learn more about your symptoms.
Based on your answers (you are age 45 or younger, no joints other than your knee are bothering you, your painful knee is swollen and there is 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:
cartilage damage and/or ligament damage
isolated arthritis of the knee (for example, spondyloarthropathy)
infection, though this is less likely (for example, Lyme disease, septic arthritis)
crystal-induced disease (such as gout, especially if you are male.