A 32-year-old man from Delhi presents with acute onset of severe right knee pain, swelling, and inability to bear weight for 2 days. He has a history of intravenous drug use. On examination, the knee is warm, erythematous, and held in 20° flexion. Vital signs: temperature 39.2°C, HR 110/min, BP 118/76 mmHg. Knee aspiration yields turbid fluid with WBC 68,000/μL (92% neutrophils), protein 5.2 g/dL, glucose 18 mg/dL (serum glucose 120 mg/dL). Gram stain shows Gram-positive cocci in clusters. Blood culture is pending. What is the most appropriate next step in management?