Rheumatology and Musculoskeletalcommon
A 68-year-old man presents with a 24-hour history of a painful, swollen right knee and is now unable to bear weight. He takes hydrochlorothiazide for hypertension and had a milder episode of pain in his left great toe two years ago that settled with anti-inflammatory tablets. His temperature is 38.1°C, the knee is warm with a moderate effusion, and active and passive movement are severely limited by pain. Blood tests show a white cell count of 13.5 x 10^9/L (4.0-11.0) and C-reactive protein of 185 mg/L (<5). Which one of the following is the most appropriate next step to establish the diagnosis?