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Melanocytic nevi and dermoscopy
Most nevi are benign; the goal is to spot the "ugly duckling" — the lesion that differs from the rest or is changing — and send it for complete excision, using the ABCDE rules and dermoscopy.
Why it matters in the exam
Questions show a changing mole, a child with a large congenital nevus, a dysplastic nevus or a blue nevus, and ask for the next step. The key decision: a suspicious lesion is removed whole (excisional biopsy) with 1–3 mm margins, not by partial biopsy. Also tested are giant congenital nevi (risk of melanoma and neurocutaneous melanosis), Spitz nevi in children, and risk factors: more than 50 nevi, atypical nevi, family history and CDKN2A. Understanding the difference between a benign nevus that changes slightly in childhood and a new or changing lesion in an adult is the heart of the question. Remember that a new nevus is relatively rare in older adults, so a new pigmented lesion after the age of forty needs particularly careful assessment. A full skin examination includes the scalp, soles, nails and mucosa, not only the lesion the patient pointed to.
Key points
- Most nevi are benign; the warning signs are asymmetry, irregular border, several colours, diameter over 6 mm and change.
- The ugly duckling sign — a lesion that differs from the rest — is more sensitive than any single criterion.
- Dermoscopy improves diagnosis; a blue-white veil, atypical network and irregular streaks point to melanoma.
- A suspicious lesion is excised whole with 1–3 mm margins so that Breslow thickness can be measured.
- A giant congenital nevus over 20 cm and many or atypical nevi raise the risk and require surveillance.
Summary table
| Nevus type | Features | Risk | Approach |
|---|---|---|---|
| Ordinary acquired nevus | Smaller than 6 mm, symmetric, uniform colour, smooth borders | Very low | Self-monitoring |
| Atypical (dysplastic) nevus | Larger than 6 mm, blurred borders, uneven colour, "fried egg" look | Marker of raised melanoma risk; does not necessarily transform | Full skin examination, body photography and dermoscopy follow-up |
| Congenital nevus | Present from birth; larger than 20 cm in adulthood counts as giant | High mainly in giant nevi, often in childhood | Follow-up; MRI when there are many satellites or a spinal location |
| Spitz nevus | Pink-red dome-shaped papule in children that grows quickly | Mostly benign; atypical Spitz is hard to separate from melanoma | Excise in adults or when atypical |
| Blue and halo nevus | Blue: uniform blue papule; halo: nevus with a white ring | Low; halo is associated with vitiligo | Assess; with a halo nevus in an adult examine the whole skin |
In the full version for subscribers
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Sources
- AAD guidelines of care for the management of primary cutaneous melanoma, 2019 — Biopsy and diagnosis
- International Dermoscopy Society consensus, 2016 — Two-step algorithm and melanoma-specific structures
- European consensus-based interdisciplinary guideline for melanoma, 2022 — Diagnosis and surveillance of patients with many naevi
Sources are given for further reading; the text is written afresh and not copied from them.
Reviewed by the Examqo AI Professor · 2026-11-06. The text may contain errors; when in doubt, check the source.