Dermatology MCQ - Infiltrative and Neoplastic Disorders - Halo naevus

A 12-year-old boy is brought to the clinic by his mother after she noticed a white spot forming around a mole on his back. On examination, there is a 6-mm, symmetrical, pink-brown central papule surrounded by a uniform, symmetric, 2-mm wide depigmented halo. The rest of his skin examination reveals two similar, but smaller, lesions on his chest. The patient is otherwise healthy and has no personal history of melanoma. What is the underlying immunopathological mechanism and the most appropriate management?

A. A cytotoxic T-cell and antibody-mediated response targeting nevomelanocytes; reassurance and clinical monitoring.
B. A loss-of-function mutation in the melanocortin-1 receptor causing localized piebaldism; perform a skin biopsy for confirmation.
C. An autoimmune destruction of melanocytes primarily driven by IgG antibodies, associated with vitiligo; refer for phototherapy.
D. A post-inflammatory hypopigmentation following regression of a lichenoid dermatitis; treat the central papule with a topical corticosteroid.
E. A phenomenon caused by local vasoconstriction and ischemia; no treatment or follow-up is required.

Correct Answer: A. A cytotoxic T-cell and antibody-mediated response targeting nevomelanocytes; reassurance and clinical monitoring.

Answer and Explanation

The correct answer is A. This question describes a classic halo naevus (Sutton's naevus). The key clinical clues are the young age of the patient, the presence of multiple lesions, the symmetrical halo, and the benign appearance of the central nevus. The hallmark pathophysiology is an inflammatory, cell-mediated immune response that targets and destroys the melanocytes within the pre-existing nevus, leading to the depigmented halo. This is often a sign of robust immune surveillance.

Why the Other Options are Incorrect:

• B. A loss-of-function mutation... causing localized piebaldism: This is incorrect. Piebaldism is a congenital, stable condition present at birth, characterized by a white forelock and fixed areas of depigmentation. It is not acquired and does not occur around a central nevus.

• C. An autoimmune destruction of melanocytes... associated with vitiligo: This is partially correct but misleading. While halo nevi and vitiligo share an autoimmune background and can coexist, the primary driver in a halo naevus is a T-cell-mediated response specifically targeted against antigens expressed by the nevus cells. Vitiligo is a more generalized process. Referring for phototherapy is not indicated for a typical halo naevus.

• D. A post-inflammatory hypopigmentation following regression of a lichenoid dermatitis: This is incorrect. While lichenoid reactions can cause hypopigmentation, they do not typically present with such a perfectly symmetrical, round halo around a pre-existing mole. The central lesion in lichenoid dermatitis would show scale or other signs of inflammation, not a simple nevus.

• E. A phenomenon caused by local vasoconstriction and ischemia: This is incorrect. The white color is due to a complete absence of melanocytes and melanin in the epidermis, not due to lack of blood flow. The skin is otherwise normal.

Additional High-Yield Information for Exams:

• Histopathology: The histology evolves over time.

  • Early Stage: A dense, band-like inflammatory infiltrate of lymphocytes and histiocytes obscuring the dermo-epidermal junction, invading and destroying the nests of nevus cells in the central papule.

  • Late/Regressed Stage: The inflammatory infiltrate resolves, the nevus cells disappear, and only a depigmented macule remains, showing a complete absence of melanocytes on Fontana-Masson or Melan-A staining.

• Differential Diagnosis: The main, critical differential is a halo phenomenon around a melanoma. Features that should raise suspicion for melanoma include:

  • Occurrence in an older adult.

  • An atypical or irregular central pigmented lesion.

  • An asymmetric or irregular halo.

• Associated Conditions & Prognosis:

  • Association with Vitiligo: Halo nevi are strongly associated with vitiligo, and their presence can be a marker for developing more generalized vitiligo.

  • Association with Autoimmunity: There is an increased incidence of halo nevi in patients with other autoimmune conditions, such as thyroid disease.

  • Prognosis: Excellent. The central nevus typically regresses completely over months to years, leaving a depigmented macule that may or may not repigment. The presence of halo nevi is not a sign of systemic illness, but rather a fascinating example of local immune surveillance.

• Management & Rationale:

  • Rationale: The goal is to correctly identify the benign lesion, rule out the sinister mimic (melanoma), and provide reassurance to avoid unnecessary procedures.

  • First-line for a classic lesion in a young person: Clinical diagnosis, reassurance, and periodic observation. The regression of the central nevus and stability of the halo are reassuring signs.

  • Indication for Biopsy/Excision: A biopsy or excision should be performed if there is any doubt about the nature of the central lesion, particularly in an adult, or if the lesion has atypical features (e.g., irregular color, asymmetry, ulceration, or an irregular halo).