A 27-year-old woman of Indian descent presents to the dermatology clinic with concerns about white patches on her hands that appeared 8 months ago. She reports symmetric bilateral involvement of her dorsal hands, perioral region, and ankles. On examination, the structure marked **A** in the diagram shows milky-white, well-demarcated depigmented macules with sharp margins and no scaling, erythema, or induration. Wood's lamp examination accentuates these lesions. She has a history of Hashimoto's thyroiditis and a family history of type 1 diabetes mellitus. Which of the following best describes the pathophysiology underlying the appearance of the structure marked **A**?
A. Selective autoimmune destruction of epidermal melanocytes leading to complete depigmentation
B. Post-inflammatory hypopigmentation due to reduced melanin synthesis in response to prior trauma
C. Reduced tyrosinase activity secondary to nutritional deficiency of copper or vitamin B12
D. Fungal infection causing disruption of melanin granules within keratinocytes
Explanation
Why "Selective autoimmune destruction of epidermal melanocytes leading to complete depigmentation" is right
The clinical presentation—milky-white (chalk-white), sharply demarcated depigmented macules with complete loss of pigmentation, symmetric bilateral distribution, absence of scaling/erythema/induration, and accentuation on Wood's lamp—is pathognomonic for vitiligo. The anchor structure A represents the hallmark lesion of vitiligo: complete depigmentation caused by selective autoimmune destruction of epidermal melanocytes. The patient's history of Hashimoto's thyroiditis and family history of autoimmune disease further support an autoimmune polyglandular syndrome context. Fitzpatrick's Dermatology (9th ed., Ch. 75) defines vitiligo as an autoimmune depigmenting disorder characterized by selective destruction of melanocytes, resulting in the chalk-white, sharply demarcated patches seen here.
Why each distractor is wrong
Post-inflammatory hypopigmentation due to reduced melanin synthesis in response to prior trauma: Post-inflammatory hypopigmentation is typically gray or tan (not chalk-white), has indistinct borders (not sharp), and the patient explicitly denies trauma or Koebner-inducing injury. The complete depigmentation and sharp demarcation rule this out.
Fungal infection causing disruption of melanin granules within keratinocytes: Fungal infections (e.g., pityriasis versicolor) present with scaling, are often pruritic, and show hypopigmentation rather than complete depigmentation. The absence of scaling and the chalk-white appearance exclude fungal etiology.
Reduced tyrosinase activity secondary to nutritional deficiency of copper or vitamin B12: Nutritional deficiencies cause diffuse hypopigmentation or hyperpigmentation, not sharply demarcated patches. The patient's screening showed normal B12 and no clinical evidence of deficiency. This mechanism does not explain the focal, symmetric, well-demarcated lesions.
