A Severe Pruritic Rash

What is the cause of this severe rash in a 60-year-old man with HIV?

A 60-year-old man with a history of HIV is seen in the ED for a severe skin eruption. He recently began a course of oral vancomycin for Clostridium difficile colitis. His other medications are nevirapine and abacavir/lamivudine which he re-started about 1 month ago after going off them temporarily. The only other symptom is the ongoing diarrhea which has slowed since starting the vancomycin.


What is the most likely diagnosis?