Amelanotic melanoma may appear as a friable, red papule. However, on histologic examination, it is composed of poorly differentiated, pleomorphic melanocytes in the dermis that demonstrate confluent growth and can show pagetoid spread in the epidermis. A lobular proliferation of vessels is not seen.
Metastatic renal cell carcinoma usually manifests as a firm, subcutaneous nodule with a predilection for the scalp. On histologic examination, it has a highly vascular stroma. However, it demonstrates poorly circumscribed sheets of “clear cells” that exhibit pleomorphism and mitoses.
Nodular Kaposi sarcoma is characterized by a proliferation of hyperchromatic spindle cells that form a vascular slit pattern with extravasated erythrocytes. Nuclear pleomorphism may be seen, and the nuclei stain positively with human herpesvirus (HHV)-8. Lobular architecture is not characteristic.
Pyogenic granulomas demonstrate a lobular proliferation of capillaries, an epidermal collarette, and an edematous stroma. Epidermal atrophy and ulceration are commonly seen.
Bacillary angiomatosis is due to bacterial infection by Bartonella species most commonly in HIV-infected patients and may manifest as red, vascular papules similarly to pyogenic granulomas. Bacillary angiomatosis shows a vascular proliferation on histologic examination, but it may be distinguished from pyogenic granuloma by more prominent neutrophilic inflammation throughout the lesion and clumps of bacilli that stain positively with Warthin-Starry or Giemsa stains.
