A 7-year-old boy from Delhi presents with fever, left eye swelling, and proptosis for 2 days. On examination, the left eye shows chemosis, lid edema, and ophthalmoplegia (restricted upward and medial gaze). Fundoscopy reveals disc edema. CT orbit shows diffuse infiltration of orbital fat with stranding. Blood culture is pending. Which of the following is the most appropriate immediate next step in management?