A 6-year-old unvaccinated boy from rural Uttar Pradesh presents with a 3-day history of sore throat, low-grade fever, and difficulty swallowing. On examination, he has a thick, adherent, grayish-white membrane over the tonsils and pharynx that does not blanch on scraping. His neck is swollen with prominent lymphadenopathy. He appears systemically well at present. What is the most appropriate immediate next step in management?