A 32-year-old woman from rural Maharashtra presents to the emergency department with a 6-hour history of severe vomiting, profuse watery diarrhoea, and abdominal cramping. Her husband reports she consumed rice that had a garlic-like odour. On examination, she is hypotensive (BP 90/60 mmHg), tachycardic (HR 118/min), and shows signs of severe dehydration. Capillary blood glucose is 95 mg/dL. A presumptive diagnosis of acute arsenic poisoning is made. What is the most appropriate immediate next step in management?