A 32-year-old man from Kolkata presents to the emergency department with acute onset of profuse watery diarrhea and vomiting for 6 hours. He reports consuming food from a street vendor near the Hooghly River 18 hours ago. On examination, he is severely dehydrated with sunken eyes, poor skin turgor, and a blood pressure of 88/54 mmHg. Stool examination shows clear, colorless fluid with a 'rice-water' appearance. Gram stain of the stool reveals curved, Gram-negative rods. Culture on thiosulfate-citrate-bile salts-sucrose (TCBS) agar shows yellow colonies. Which toxin produced by the causative organism is primarily responsible for the massive fluid loss in this patient?