A 35-year-old woman presents with fever, facial edema, and a widespread erythematous maculopapular rash that began 3 weeks after starting carbamazepine for newly diagnosed epilepsy. Laboratory investigations reveal: WBC 12,000/μL with 15% atypical lymphocytes, ALT 180 U/L, AST 165 U/L, and mild lymphadenopathy on examination. Which of the following is the most appropriate immediate management?