Dermatology MCQ - Infiltrative and Neoplastic Disorders - atypical genital melanocytic naevus

A 25-year-old woman is concerned about a pigmented lesion on her labia majora. It has been present for several years and is asymptomatic. On examination, there is a 7-mm, dark brown, slightly raised, symmetrical plaque with well-defined but irregular, geographic borders. The surface is verrucous. Dermoscopy shows a homogeneous, structureless brown pattern with comedo-like openings. What is the most likely diagnosis and the most appropriate management?

A. Melanoma; perform a wide local excision with 1-2 cm margins.
B. Genital melanotic macule; provide reassurance and monitor.
C. Vulval melanosis; provide reassurance and consider baseline photography.
D. Genital melanocytic naevus (atypical genital nevus); perform a complete but conservative excision for histological diagnosis.
E. Seborrheic keratosis; offer cryotherapy or shave removal for cosmetic reasons.

Correct Answer: D. Genital melanocytic naevus (atypical genital nevus); perform a complete but conservative excision for histological diagnosis.

Answer and Explanation

The correct answer is D. This question describes a classic atypical genital melanocytic naevus. The key is recognizing that melanocytic nevi in the genital area often have clinical features (size >6mm, dark color, irregular borders, verrucous surface) that would be highly suspicious for melanoma in other locations. However, in the genital region, these features can be part of the spectrum of a benign, but histologically atypical, nevus. The dermoscopic description of a homogeneous pattern with comedo-like openings is also consistent with this entity. Because the clinical and dermoscopic features overlap significantly with melanoma, the standard of care is complete excision for definitive histological diagnosis.

Why the Other Options are Incorrect:

• A. Melanoma; perform a wide local excision...: While melanoma must be ruled out, jumping to a diagnosis of melanoma and performing a wide excision is premature without a histological confirmation. The lesion's long-term stability is a reassuring feature. The initial step is a diagnostic, not therapeutic, excision.

• B. Genital melanotic macule / C. Vulval melanosis: These terms are often used interchangeably and describe benign, flat (macular) hyperpigmentation due to increased melanin in keratinocytes without a proliferation of melanocytes. The lesion described is raised (a plaque) and has a verrucous surface, which is inconsistent with a simple macule.

• E. Seborrheic keratosis; While the verrucous surface and comedo-like openings on dermoscopy can mimic a seborrheic keratosis, the location (genitalia) is atypical for a seborrheic keratosis. Furthermore, the significant pigment and the clinical concern in this location make a melanocytic lesion more likely, and a shave removal would be inadequate for the full histological assessment needed.

Additional High-Yield Information for Exams:

• Histopathology: Genital melanocytic nevi, particularly the "atypical" variant, often show specific features. They can have a compound or junctional pattern with large, confluent nests of melanocytes at the dermo-epidermal junction, sometimes with a "pagetoid" scatter of melanocytes into the upper epidermis. This pagetoid scatter can be a benign feature in this specific context (a major diagnostic pitfall) and must be correlated with the overall architecture and cytology to distinguish it from melanoma.

• Differential Diagnosis: This is critical. The differential for a pigmented genital lesion includes:

  • Atypical Genital Naevus (as above)

  • Melanoma: The diagnosis to rule out. Features more suggestive of melanoma include rapid change, ulceration, bleeding, and pain.

  • Vulval Melanosis/Macule: Flat, irregularly shaped, but non-palpable macular hyperpigmentation.

  • Lentigo: Similar to a macule but may show rete ridge elongation on histology.

• Prognosis: Atypical genital nevi are considered benign, but they are often excised due to diagnostic difficulty. There is no clear evidence that they confer a significantly increased risk of melanoma elsewhere.

• Management & Rationale:

  • Rationale: The primary rationale is diagnostic certainty. The significant clinical and histological overlap with melanoma makes observation risky. Excision provides a complete specimen for the pathologist to evaluate architecture and rule out malignancy.

  • First-line: Complete but conservative excision with a narrow (1-2 mm) margin. This is both diagnostic and therapeutic if the lesion is benign.

  • Monitoring: Is generally not recommended for lesions with suspicious features because of the inability to reliably distinguish them from melanoma clinically.

  • Pathology Communication: It is often helpful to communicate the lesion's location to the pathologist, as the histological criteria for diagnosis differ from those used for nevi on trunk and extremities.