A 24-year-old woman presents to the emergency department with a 3-day history of fever, malaise, and painful oral ulcers. On examination, she has extensive erythematous macules and papules on her trunk and extremities, with some lesions showing central blistering. Her oral mucosa, conjunctivae, and genital area are involved. She reports starting cotrimoxazole for a urinary tract infection 5 days ago. Temperature is 38.5°C, blood pressure 110/70 mmHg. Skin biopsy shows full-thickness epidermal necrosis with subepidermal blister formation. What is the most appropriate immediate management?