Pseudoxanthoma Elasticum Chicken-Skin Neck

A 32-year-old woman presents with a three-year history of progressive skin changes. On examination, the structure marked **A** — multiple yellow-orange papules coalescing into cobblestone plaques on the lateral neck — is noted bilaterally. She also reports recent visual distortion with metamorphopsia, two episodes of hematemesis requiring transfusion, and claudication-like calf pain. Fundoscopy reveals angioid streaks and choroidal neovascularization. Skin biopsy shows fragmented, calcified, disorganized elastic fibers in the mid-dermis. Genetic testing confirms biallelic ABCC6 mutations. Which of the following best describes the pathophysiology underlying the cutaneous findings marked **A**?