A 28-year-old woman from rural Maharashtra presents to the emergency department with severe abdominal pain, tachycardia (HR 110/min), and confusion. She reports a 3-day history of colicky abdominal pain without diarrhea, followed by acute onset of neuropsychiatric symptoms including anxiety and disorientation. On examination, she is hypertensive (BP 160/95 mmHg) and has proximal muscle weakness. Urine dipstick is normal, but fresh urine left standing in sunlight turns dark red-brown over 2 hours. Serum sodium is 125 mEq/L (hyponatremia). Which of the following is the most likely diagnosis?