A 77 year old retired man presented in a wheelchair to the emergency department with severe, constant pain and progressive swelling of the left knee. His symptoms had suddenly occurred two days before at night after he had gone to bed feeling well. He could not recall any injury to the knee. There was no history of joint disease or joint surgery. His medical history was unremarkable, and he was not taking any regular medication. He was not aware of any family history of arthritis. He smoked five cigarettes a day and drank two glasses of wine a day.

On examination, the patient did not have a fever and had a heart rate of 82 beats/min and a blood pressure of 130/75 mm Hg. His body mass index was within normal limits. His left knee was red, hot, swollen, and tender with minimal range of motion. No other joint involvement was detected and there were no extra-articular findings. Synovial fluid was taken from the joint (fig 1⇓) and studied under compensated polarised light microscopy (fig 2⇓). A radiograph of the knee was also taken (fig 3⇓). All other system examinations were normal.

Blood tests showed an erythrocyte sedimentation rate of 55 mm/h (normal <25 mm/h) and a C reactive protein concentration of 1.2 mg/dl (normal <0.5 mg/dl). Peripheral white blood cell count, platelet count, and haemoglobin concentration were within normal limits. Kidney and liver function parameters, and electrolytes and serum urate levels, were normal.