A 4-year-old girl from Uttar Pradesh with incomplete DPT vaccination presents to the pediatric ward on day 5 of illness with a history of sore throat, fever, and progressive difficulty in swallowing. She was treated with antibiotics at a primary health center 2 days ago. On examination, she has a thick pseudomembrane covering both tonsils, cervical lymphadenopathy with marked neck edema ('bull neck' appearance), and a heart rate of 120/min with a blood pressure of 95/60 mmHg. Her respiratory rate is 32/min. Throat culture and PCR for *Corynebacterium diphtheriae* are pending. She has already received diphtheria antitoxin 50,000 units IV 6 hours ago. What is the most important next step in her management?