A 32-year-old woman presents with progressive loss of skin pigmentation over her hands, feet, and around her mouth over the past 6 months. On examination, the lesions show **sharp demarcation from normal skin** (marked **B** in the diagram), and appear chalk-white. Under Wood's lamp, the lesions appear bright bluish-white with enhanced contrast. She has a history of Hashimoto thyroiditis diagnosed 2 years ago. Based on the characteristic sharp demarcation and Wood's lamp findings, what is the most likely pathogenic mechanism underlying this condition?