<s>[INST] context: Acute monoarticular arthritis in association with linear calcification of the cartilage of the knee (chondrocalcinosis) suggests the diagnosis of pseudogout, a form of calcium pyrophosphate dihydrate deposition (CPPDD) disease. In its acute manifestation, the disease resembles gout. Positively birefringent crystals (looking blue when parallel to the axis of the red compensator on a polarizing microscope) can be demonstrated in joint fluid, although careful search is sometimes necessary. Serum uric acid and calcium levels are normal, as are rheumatoid factor and ANA. Pseudogout is about half as common as gout but becomes more common after the age of 65. CPPDD disease is diagnosed in symptomatic patients by characteristic x-ray findings and crystals in synovial fluid. Pseudogout is treated with NSAIDs, colchicine, or steroids. Arthrocentesis and drainage with intra-articular steroid administration is also an effective treatment. Linear calcifications or chondrocalcinosis are often found in the joints of elderly patients who do not have symptomatic joint problems; such patients do not require treatment. [/INST] question: A 65-year-old man develops the onset of severe right knee pain over 24 hours. The knee is red, swollen, and tender. The patient does not have fever or systemic symptoms; he has never had severe joint pain before. Plain film of the knee shows linear calcification of the articular cartilage without destructive change. Definitive diagnosis is best made by which of the following? | options: A: Serum uric acid B: Serum calcium C: Arthrocentesis and identification of positively birefringent rhomboid crystals D: Rheumatoid factor | answer: C </s>