You are seeing an 81-year-old male in a GP setting who is complaining of a rash on his scalp. The patient reported that this rash began as a small pimple looking lesion on the scalp that became paler and scalier over the course of a few days. The hair around it is now discoloured and brittle, breaking off a few millimetres above the scalp surface. The patient is also complaining that this rash is itchy and that the lesion is now spreading. On examination you note numerous papules in the shape of a ring. What is the diagnosis?