A 41-year-old man was referred with bilateral swollen knees. He reported a 6-year history of pain and swelling in the knees and feet. Past medical history revealed psoriasis and a recent diagnosis of hypertension. He had never smoked, drank less than 10 units of alcohol per week and was not on medication.

On examination he had massive bilateral knee joint effusions and knee movement was restricted, with flexion not beyond 110° in either knee. Skin examination demonstrated plaque psoriasis and ichthyosis. Blood tests showed C-reactive protein of 8 mg/l, erythrocyte sedimentation rate of 17 mm/h and urate at 0.32 mmol/l. Antinuclear antibody screen and rheumatoid factor were negative. Further laboratory tests were unremarkable. Approximately 400 ml of fluid was aspirated from the right knee and 100 ml from the left, which was aseptic but contained high numbers of mixed leucocyte populations. Magnetic resonance imaging of the knees …