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Case Reports

Abstract

Proliferative nodules can arise as nodules or plaques within congenital melanocytic nevi and typically represent a benign biologic process that often regresses. These nodules often present in young children and occur within larger congenital melanocytic nevi. Proliferative nodules can have diverse histologic patterns and typically show maturation with minimal cytologic atypia and mitoses. A subset of proliferative nodules, however, may show atypical features including increased cellularity, cytologic atypia, inflammation, and ulceration, which make them diagnostically challenging to distinguish from melanoma. Herein, we discuss a case of a 1-year-old male, without significant past medical history, who presented with two new, tender nodules with superficial ulceration and heme crust within a 13 x 12 cm congenital melanocytic nevus on the right medial shin, clinically concerning for a proliferative nodule versus melanoma. A shave biopsy showed an intradermal nevus with congenital features. The biopsy had areas of increased cellularity with ulceration and a dense inflammatory infiltrate composed of neutrophils, lymphocytes, and many eosinophils. HMB-45 was positive, predominately with superficial staining, and Ki-67 was less than 1%. A localized hypersensitivity reaction secondary to an arthropod bite, clinically simulating a proliferative nodule within a congenital nevus, was favored. Arthropod bite hypersensitivity reactions can result in acute and clinically concerning presentations. When arising as a nodule within a large congenital melanocytic nevus, it can be clinically mistaken for a proliferative nodule or melanoma.  This case highlights the importance of clinico-pathologic correlation when evaluating these lesions to guide us towards the correct diagnosis.