A 7-year-old unvaccinated boy from rural Maharashtra presents with a 3-day history of sore throat, low-grade fever (38.2°C), and progressive difficulty swallowing. On examination, he has a thick, greyish-white pseudomembrane covering both tonsils and extending into the pharynx. The membrane does not bleed on gentle scraping. His neck shows marked lymphadenopathy with "bull neck" appearance due to soft tissue edema. A throat swab is sent for culture. Which toxin produced by the causative organism is responsible for the systemic manifestations observed in this patient?