A 7-year-old child is brought to the clinic with complaints of an itchy, red rash on both hands.The parent reports that the child recently started using a new brand of scented hand soap.Upon examination, the nurse observes erythematous, dry, and scaly patches on the dorsum of both hands, without vesicles or oozing.The child has no significant medical history and is otherwise in good health.Which of the following is the most likely cause of the child's skin condition?