A 32-year-old woman from Delhi presents on day 5 of fever with severe abdominal pain, persistent vomiting, and a platelet count of 45,000/μL. Her haemoglobin is 14 g/dL, haematocrit 42%, and urine output is 800 mL in 24 hours. NS-1 antigen is positive. What is the most appropriate immediate next step in management?