Derm Dx: Rectal rash preceded by pain in an elderly man

A 71-year-old man presents for evaluation of a rash affecting his rectum and right buttock. The rash was preceded by pain at the site that radiated down his right leg. Medical history is positive for colon cancer resected 5 years earlier. He denies prior history of a similar rash and denies recurrent fever blisters or genital ulcers. Two years earlier, ...