A 4-year-old girl with diphtheria is on day 5 of antitoxin and IV penicillin therapy. She remains afebrile and the pharyngeal membrane has begun to separate. However, her mother reports that the child has developed weakness in both legs over the past 24 hours and is unable to climb stairs. On neurological examination, you note symmetrical lower limb weakness (grade 3/5) with diminished reflexes. What is the most appropriate next step in management?