Painless plaques on the lower back

A man in his 40s presented with painless zosteriform erythematous plaques covered with white scales on the left lower back for the past two weeks. He had a 10 year history of psoriasis and had been receiving subcutaneous injections of secukinumab at a fixed dose of 300 mg every four weeks for more than two years without relapse. In the previous four weeks, he had been diagnosed with herpes zoster, which presented as a unilateral distribution of oedematous erythema and clustered blisters accompanied by a paroxysmal pins and needles sensation on the left lower back. He was prescribed antivirals for one week. The blisters dried and crusted off over 10 days without post-herpetic neuralgia. Two days later, red lesions with white scales appeared atop the healing remnants of the herpes zoster. Physical examination showed erythematous patches or plaques 1-3 cm in size and covered with white scales. …