Atopic Dermatitis (Eczema) — Childhood

A 9-year-old boy presents with a 3-year history of intensely pruritic skin lesions. On examination, the rash is predominantly localized to the **antecubital and popliteal fossae**, as marked by **D** in the diagram. His mother reports he had facial and scalp involvement when he was 18 months old, which has now resolved. He has a family history of asthma and allergic rhinitis. Which of the following best explains the characteristic distribution pattern shown at **D**?