A 6-year-old unvaccinated child from rural Maharashtra presents with a 3-day history of sore throat, low-grade fever, and progressive difficulty swallowing. On examination, there is a greyish-white pseudomembrane covering the tonsils and pharynx that bleeds on attempted removal. The child appears toxic with cervical lymphadenopathy and mild stridor. What is the most appropriate immediate next step in management?