You have seen a forty-year-old female in your clinic who has presented with an acute onset of right knee pain. She denied any trauma and states that she is in a lot of pain. You examined the area and note that there is obvious swelling, redness and it is very tender on palpation. You decided to aspirate the knee joint and send the sample for joint fluid microscopy and culture. You have just received the results which report that the sample shows calcium pyrophosphate crystals. What is the diagnosis?