Dermatology MCQ - Infiltrative and Neoplastic Disorders - Recurrent melanocytic naevus

A 28-year-old woman had a pigmented lesion on her back shave-biopsied 6 months ago. The pathology was reported as a benign compound melanocytic nevus. She now presents concerned that the "mole is growing back." On examination, within the well-healed scar, there is a 5 mm area of variegated brown pigmentation. The pigmentation is confined to the area of the scar and does not extend beyond the original biopsy site. Dermatoscopy shows a symmetrical, starburst-like pattern of pigment with peppering and dots at the periphery. What is the most likely diagnosis and the most appropriate management?

A. Recurrent melanocytic nevus; complete re-excision with 2-mm margins is mandatory to rule out malignancy.
B. Melanoma arising in a recurrent nevus; perform a wide local excision with 1-cm margins.
C. Recurrent melanocytic nevus; reassure the patient and monitor clinically, as this is a benign phenomenon.
D. Post-inflammatory hyperpigmentation; treat with a topical bleaching cream and sun protection.
E. Traumatic tattoo from the surgical procedure; no further treatment is needed.

Correct Answer: C. Recurrent melanocytic nevus; reassure the patient and monitor clinically, as this is a benign phenomenon.

Answer and Explanation

The correct answer is C. This question describes the classic presentation of a recurrent (or persistent) melanocytic nevus. The key clues are the history of a previous biopsy (especially a shallow shave biopsy), the location of the repigmentation strictly within the confines of the scar, and the characteristic dermatoscopic pattern (often a starburst, peppering, or globular pattern confined to the periphery). This is a benign process where residual nevus cells from the original lesion proliferate within the healing scar tissue.

Why the Other Options are Incorrect:

• A. Recurrent melanocytic nevus; complete re-excision... is mandatory: This is incorrect. While re-excision is sometimes performed for peace of mind or if the appearance is highly atypical, it is not mandatory for a classic recurrent nevus. The clinical and dermoscopic features are often sufficient for diagnosis, and monitoring is an accepted standard of care.

• B. Melanoma arising in a recurrent nevus: This is incorrect. While true malignant recurrence is a concern, it is rare. Features that would suggest melanoma include rapid growth, pigment extending beyond the scar borders, and a dermatoscopic pattern of melanoma (e.g., atypical pigment network, irregular dots/globules). The described features are classic for benign recurrence.

• D. Post-inflammatory hyperpigmentation (PIH): This is incorrect. PIH appears as a flat, diffuse, brown macule without specific structures. The dermatoscopic description of a "starburst-like pattern" and "peppering" indicates a proliferation of melanocytes, not simply dermal melanin deposition.

• E. Traumatic tattoo: This is incorrect. A traumatic tattoo from a surgical blade would present as irregular, blue-black particulate matter implanted in the skin, not as a symmetrical, brown, starburst pattern of repigmentation.

Additional High-Yield Information for Exams:

• Histopathology: The histology of a recurrent nevus is characteristic and crucial to distinguish it from melanoma. It shows:

  • A scar in the superficial dermis.

  • A proliferation of melanocytes confined to the epidermis (junctional nests and single cells) directly above the scar.

  • There is often a sharp lateral circumscription; the melanocytic proliferation stops abruptly at the edge of the scar.

  • Melanocytes can show reactive atypia (enlarged nuclei) due to the inflammatory and reparative environment, but they lack the severe, random atypia of melanoma.

  • Crucially, the melanocytes do not extend down adnexal structures beyond the scar, a feature that helps differentiate it from melanoma.

• Differential Diagnosis: The primary and most important differential is recurrent melanoma. Key features favoring melanoma include asymmetry, extension of pigment beyond the scar, and a history of an incompletely excised or dysplastic original lesion.

• Prognosis: A recurrent melanocytic nevus is a benign process. It does not have an increased risk of transforming into melanoma.

• Management & Rationale:

  • Rationale: The goal is to avoid unnecessary re-excision of a benign lesion while remaining vigilant for the rare case of true malignant recurrence.

  • First-line for a classic presentation: Clinical and dermoscopic monitoring with reassurance. Baseline clinical and dermoscopic photographs are extremely helpful for documenting stability.

  • Indication for Re-excision: Re-excision should be considered if:

    • The original biopsy was for a lesion with concerning features (e.g., severe atypia).

    • The clinical or dermoscopic appearance is atypical and not classic for a benign recurrence.

    • The patient is highly anxious.

    • The lesion demonstrates significant change on follow-up.