A 42-year-old male farmer from Punjab presents to the emergency department with a 3-day history of severe vomiting, profuse watery diarrhea, and abdominal pain. His wife reports that he consumed water from an old well on their farm. On examination, he is hypotensive (BP 80/50), tachycardic (HR 118/min), and has signs of severe dehydration. His skin shows hyperpigmentation over the palms and soles with hyperkeratosis. Laboratory investigations reveal elevated serum creatinine (2.8 mg/dL) and elevated liver enzymes. A 24-hour urine collection shows elevated arsenic levels (150 µg/L; normal <50 µg/L). What is the most appropriate immediate management?