A 34-year-old man presents with dozens of soft, skin-colored to slightly pink papules and nodules scattered over his trunk, back, and proximal limbs. Individual lesions range from 3 mm to 2 cm, are non-tender and rubbery. When firm pressure is applied over the apex of these lesions marked **A** in the diagram, they characteristically invaginate inward like a button through a buttonhole. He also has six café-au-lait macules ≥15 mm, axillary freckling, and a plexiform neurofibroma. Genetic testing confirms a pathogenic variant in NF1 on chromosome 17q11.2. Which of the following best explains the clinical presentation of the lesions marked **A**?