A 6-year-old unvaccinated boy from rural Maharashtra presents with a 3-day history of sore throat, low-grade fever (38.2°C), and progressive dysphagia. On examination, he has a thick, adherent, greyish-white membrane over the posterior pharynx extending to the soft palate and tonsils. His neck shows mild lymphadenopathy and soft tissue edema. Respiratory rate is 28/min. A nasopharyngeal swab is sent for culture. Which of the following is the most appropriate immediate management?