A 42-year-old woman from rural Maharashtra presents to the emergency department with a 3-day history of severe abdominal pain, profuse watery diarrhea, and vomiting. Her husband reports she consumed water from a hand pump near an old pesticide storage site. On examination, she is hypotensive (BP 80/50), tachycardic (HR 118/min), and has signs of severe dehydration. Her skin shows diffuse hyperpigmentation and hyperkeratosis on the palms and soles. Laboratory investigations reveal: hemoglobin 16 g/dL, WBC 12,000/μL, serum creatinine 2.8 mg/dL, and elevated liver enzymes. Urine shows dark brown discoloration. What is the most likely diagnosis?