Pseudoxanthoma Elasticum Neck

A 48-year-old woman presents with asymptomatic yellowish bumps on her neck and flexural areas that have progressively worsened over the past 15 years. The structure marked **A** in the diagram shows the characteristic appearance of these lesions. Skin biopsy reveals fragmented and calcified elastic fibres. She also reports recent visual disturbance in the left eye, and fundoscopy shows retinal angioid streaks with choroidal neovascularisation. Genetic testing confirms compound heterozygous ABCC6 mutations. Which of the following best explains the pathophysiology underlying the cutaneous findings marked **A**?