A 34-year-old woman presents with bilateral progressive central visual distortion and a history of yellowish cobblestone-like plaques on the lateral neck, antecubital fossae, and axillae. She has a remote history of upper GI bleeding at age 22. Fundus examination reveals bilateral irregular, jagged, reddish-brown lines originating from the peripapillary region and radiating outwards in a spoke-like fashion, deeper than the retinal vessels. The structure marked **A** in the diagram represents these characteristic findings. Fundus fluorescein angiography demonstrates window defects along these streaks due to atrophic RPE overlying calcified breaks. Based on the clinical presentation and the nature of the structure marked **A**, what is the most likely underlying systemic diagnosis?