A 38-year-old woman on allopurinol for gout presents with a 2-week history of fever (39°C), a widespread pruritic rash involving the face, trunk, and flexural areas, and cervical lymphadenopathy. Laboratory investigations show WBC 12,000/μL with 18% eosinophils, elevated transaminases (AST 120 U/L, ALT 140 U/L), and atypical lymphocytes on blood smear. Skin biopsy shows perivascular lymphocytic infiltration with eosinophils. Which of the following is the most appropriate immediate management?