A 75-year-old man with a history of chronic obstructive pulmonary disease (COPD) and colon cancer presents with a 3-day history of an intensely pruritic and painful rash on his left mid-back.
He states that it began as a tingling area before he began to develop multiple blisters. He does not have any other sites of involvement.
On examination, there is a unilateral erythematous rash consisting of papules and vesicles in a dermatomal distribution on the back.
What could be the diagnosis ?