A 7-year-old child from Delhi presents with a 3-day history of sore throat, dysphagia, and low-grade fever. On examination, there is a thick, adherent, grayish-white pseudomembrane over the soft palate and pharynx that bleeds on attempted removal. The child has not received any diphtheria immunization. Rapid antigen test is positive for Corynebacterium diphtheriae. What is the most appropriate immediate next step?