Prostatitis and pruritus

A 47-year-old man had severe itching that started the previous day and kept him up all night. We examined his rash (Figure ) as we inquired about his history. He had no previous skin problems and no known allergies. He had no fever, pain, or malaise.

We discovered that a month ago he had started taking 1 trimethoprim-sulfamethoxazole double-strength tablet twice daily for chronic prostatitis. He was told he would need to take the medication for 3 months. Other than prostatitis, he had no other medical problems and was not taking any other medications.

FIGURE

What is the diagnosis?
What is the best course of management?