A 60-year-old man with a 2-year history of rheumatoid arthritis, currently treated with methotrexate 15mg weekly and folic acid, presents with increasing pain and swelling in his right knee over the past 3 days. He denies trauma. On examination, the knee is warm, swollen, and tender, with a restricted range of motion. Aspiration of the knee joint yields cloudy synovial fluid with a white cell count of 75,000/mm³, predominantly neutrophils. No crystals are seen on polarised light microscopy. What is the most appropriate next step in management?