A 32-year-old man presents to the emergency department with a 2-day history of severe right knee pain, swelling, and inability to bear weight. He has a fever of 38.5°C. On examination, the knee is warm, erythematous, and there is a large effusion. He denies recent trauma or surgery. Synovial fluid analysis shows WBC 85,000/μL (95% neutrophils), protein 6.2 g/dL, glucose 18 mg/dL (serum glucose 120 mg/dL), and Gram stain shows Gram-positive cocci in clusters. Blood cultures are pending. What is the most appropriate immediate next step in management?