A 35-year-old man from Mumbai is admitted to the ICU with a 7-day history of fever, widespread blistering, and skin erosions affecting approximately 45% of his body surface area. He has involvement of the oral mucosa, conjunctivae, and genital region with severe pain. He developed these symptoms 3 weeks after starting phenytoin for newly diagnosed epilepsy. On examination, he has flaccid blisters, positive Nikolsky sign, and erythematous patches with areas of full-thickness epidermal detachment. His temperature is 40.1°C, heart rate 128/min, blood pressure 92/58 mmHg, and respiratory rate 28/min. Laboratory findings: WBC 14,200/μL, albumin 2.8 g/dL, creatinine 2.4 mg/dL, and blood cultures are pending. What is the most likely diagnosis, and what is the primary reason for the poor prognosis in this case?