This biopsy came from a teenage girl to evaluate an irregular nevus. While it is similar to a compound nevus at low magnification, you may appreciate a dimorphous proliferation of melanocytes in the dermis. At higher magnification, there is a population of severely atypical melanocytes with large irregular nuclei, abundant cytoplasm, and mitotic activity. The tumor extends to the base of the biopsy and lacks of maturation with descent into the dermis. I initially considered that the melanoma may arising in an associated nevus, although the smaller melanocytes in the dermis also show atypia, hyperchromasia, and mitotic activity. Within the epidermis there is increased upward migration of individual melanocytes, extending to the granular layer. All of these features are consistent with malignant melanoma.
Melanoma and melanocytic tumors of uncertain malignant potential in children, adolescents and young adults–the Stanford experience 1995-2008. Burk DR et al. Pediatr Dermatol. 2010 May-Jun ;27(3):244-54. Epub 2010 Apr 9