Answer: Blue nevus

Histology: none provided

Discussion: The blue nevus is a relatively common lesion that frequently affects the dorsal aspect of the hands and feet, the buttocks, the face or the scalp. Clinically, blue nevi are dome-shaped, with a blue or blue-black appearance. Histologically, these lesions consist of a dermal proliferation of pigmented dendritic cells with a characteristic bipolar morphology. This proliferation may extend into the deep reticular dermis and is usually oriented parallel to the skin surface. A fibroblastic and collagenous stroma surrounds the dendritic cells, and numerous heavily pigmented macrophages are often present. Mitotic figures and atypical cytologic features are not appreciated, which helps to distinguish blue nevi from malignant lesions such as desmoplastic melanoma.