A 28-year-old woman presents to the emergency department with widespread painful blistering and mucosal erosions that started 3 days after starting trimethoprim-sulfamethoxazole for a urinary tract infection. Examination reveals erythematous macules and targetoid lesions involving 25% of body surface area, with involvement of oral, ocular, and genital mucosa. Temperature is 38.5°C. What is the most appropriate immediate next step in management?