Pityriasis Rosea — Herald Patch + Christmas Tree Distribution

A 22-year-old woman presents with a 2-week history of a widespread eruption on her trunk and proximal arms. She recalls a single large salmon-pink plaque on her chest 10 days prior, which she initially thought was a fungal infection. On examination, she has multiple small oval pink macules and papules distributed symmetrically over her back. The lesions are arranged in a characteristic pattern as shown by **C** in the diagram. Which of the following best explains the anatomical basis for this distinctive distribution pattern?