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 the pharynx and tonsils that bleeds when attempts are made to remove it. Cervical lymphadenopathy is marked, giving a "bull neck" appearance. A throat swab is sent for culture. Which organism is most likely responsible, and what is the pathogenic mechanism of systemic toxicity?