A 19-year-old man has a pigmented skin lesion. Which of the following characteristics suggests a dysplastic nevus (atypical mole) rather than a benign acquired nevus?

Dysplastic nevi and benign acquired nevi are both most common on sun-exposed areas such as the back. Atypical moles can occur on the scalp, breasts, and buttocks, rare areas for benign acquired nevi. Both lesions are usually associated with similar lesions (10-40 on average for benign nevi, often > 100 in the case of dysplastic nevi). Dysplastic nevi are larger (> 6 mm) and have irregular pigmentation and borders.