A 19-year-old man develops a rash in the groin area. On examination, it is a large well-demarcated area of tan-brown discoloration around his left inguinal area. There is some scaling of the lesion when brushed with a tongue depressor. Which of the following is the most appropriate initial diagnostic test?

A KOH preparation is useful when performed on scaling skin lesions, when a fungal etiology is suspected. The scraped scales are placed on a microscope slide, treated with one or two drops of KOH solution, and examined for hyphae, pseudohyphae, or budding yeast. A Tzanck smear is a cytologic technique for the diagnosis of herpes virus infection from vesicles, and diascopy is to assess whether a skin lesion will blanch with pressure.