A 14-year-old girl presents with a solitary lesion on her upper back. Dermoscopy reveals a 4 mm evenly pigmented nevus surrounded by a sharply demarcated, symmetric, chalk-white depigmented zone approximately 2 mm wide. The structure marked **B** in the diagram is most consistent with this clinical presentation. Which of the following best describes the pathophysiologic mechanism underlying this lesion?
A. Loss of melanocyte stem cells due to genetic mutation in the MITF gene
B. Melanoma with spontaneous regression and surrounding inflammatory response
C. Fungal infection causing depigmentation around a pre-existing nevus
D. CD8+ cytotoxic T lymphocyte-mediated autoimmune destruction of melanocytes
Explanation
Why CD8+ cytotoxic T lymphocyte-mediated autoimmune destruction is right
The structure marked B (halo nevus or Sutton nevus) is defined by an autoimmune T-cell mediated reaction against melanocytic antigens. CD8+ cytotoxic T lymphocytes recognize melanocytic antigens such as MART-1/Melan-A, gp100, and tyrosinase, leading to progressive destruction of both the central nevus and surrounding normal melanocytes. This produces the characteristic symmetric, sharply demarcated depigmented halo. The clinical presentation—a young adolescent with a typical nevus, symmetric halo, and chalk-white depigmentation—is pathognomonic for benign halo nevus, confirming the autoimmune mechanism (Bolognia Dermatology 5e; Fitzpatrick's 9e Chapter 122).
Why each distractor is wrong
Melanoma with spontaneous regression: While amelanotic melanoma with regression can mimic halo nevus, this presentation is reassuring for benign halo nevus: the patient is a child, the nevus is evenly pigmented, the halo is symmetric, and there are no atypical features. Melanoma-associated regression typically occurs in adults and presents with asymmetry or irregular borders—red flags requiring biopsy.
Loss of melanocyte stem cells due to MITF mutation: This describes congenital or syndromic pigmentary disorders (e.g., Waardenburg syndrome), not the acquired, autoimmune mechanism of halo nevus. MITF mutations cause developmental pigment abnormalities, not T-cell mediated destruction.
Fungal infection causing depigmentation: Tinea versicolor and other fungal infections cause irregular, non-symmetric depigmentation and are KOH-positive. They do not produce the sharply demarcated, symmetric halo around a central nevus characteristic of Sutton nevus.
High-YieldNEET PG
Halo nevus in a child or adolescent with a typical nevus and symmetric halo = benign autoimmune reaction; halo nevus in an adult with atypical features = always biopsy to exclude regressing melanoma.
Bolognia Dermatology 5e; Fitzpatrick's 9e Chapter 122
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