Biopsy from a mole on the foot shows cytologic atypia of melanocytes and diffuse epidermal infiltration by anaplastic cells, which are also present in the papillary and reticular dermis. The most likely diagnosis is:

Depending upon the depth of invasion below the granular cell layer in millimeter, Clark has described 5 levels: Level 1: Malignant melanoma cells confined to the epidermis and it's appendages. Level 2: Extension in to the papillary dermis Level 3: Extension of tumor cells in to the interface between papillary and reticular dermis. Level 4: Invasion of reticular dermis Level 5: Invasion of the subcutaneous fat In the question histology shows cytologic atypia of melanocytes and diffuse epidermal infiltration by anaplastic cells in the reticular dermis. So the answer is Melanoma, Clarks level IV. Ref: Harsh Mohan Pathology, 5th Edition, Page 812; Harissons Internal Medicine, 18th Edition, Page 87.