Vitiligo Symmetric Depigmentation

A 24-year-old woman of South Asian descent (Fitzpatrick type V) presents with a 18-month history of progressive white patches on her face, hands, and feet. She reports that her mother has Hashimoto thyroiditis and her sister has type 1 diabetes. On examination, the structure marked **A** in the diagram—symmetric chalk-white depigmented macules and patches with sharply demarcated borders—are distributed over the dorsal hands, periocular region, perioral area, elbows, and knees. The lesions are completely asymptomatic, and Wood lamp examination shows brilliant blue-white fluorescence. She has recently been diagnosed with autoimmune hypothyroidism and pernicious anemia. Which of the following best characterizes the pathophysiology underlying the appearance of the structure marked **A**?