A 24-year-old Indian woman presents to the emergency department on day 10 of carbamazepine therapy (initiated for newly diagnosed epilepsy) with fever, malaise, and painful oral ulcers. Examination reveals dusky atypical target lesions on the trunk and extremities, a positive Nikolsky sign, and the finding marked **B** in the diagram — hemorrhagic crusts on the lips. She has not undergone HLA-B*1502 screening prior to drug initiation. Which of the following is the MOST critical immediate management step to reduce mortality in this patient?