A 32-year-old woman presents to her general practitioner complaining of a persistent, intensely itchy rash that started a few weeks ago. She reports that it began on her wrists and ankles and has now spread to her elbows and knees. She denies any new medications, travel, or changes in personal care products. On examination, you observe symmetrical, excoriated papules and plaques on her wrists, elbows, knees, and upper back. Some of the papules have small blisters. Which of the following is the most appropriate initial investigation?