A 28-year-old man presents with fever, generalized lymphadenopathy, and a pruritic maculopapular rash over the trunk and extremities that began 3 weeks after starting phenytoin for newly diagnosed seizures. Laboratory investigations show WBC 12,000/μL with atypical lymphocytes, elevated transaminases (ALT 280 U/L), and mild thrombocytopenia. Which of the following is the most appropriate immediate management?