A 45-year-old woman with lifelong high myopia (−16 D, axial length 30.4 mm) presents with progressive central vision loss over 5 years. Fundus examination reveals a large posterior pole outpouching with a circumscribed pigmented arc delineating its margin—the structure marked **A** in the diagram. B-scan ultrasound confirms local posterior ectasia with a radius of curvature shorter than the surrounding globe. OCT demonstrates choroidal thickness of 48 µm with severe outer-retinal thinning and loss of the ellipsoid zone. Which of the following best describes the pathophysiologic basis of the staphyloma edge marked **A**?