Pigmented Lesion on the Back

REPORT OF A CASE A 72-year-old white man presented with a pigmented plaque on his upper back. The lesion had been present for more than 30 years and its size had gradually increased; it was asymptomatic. On clinical examination, there was a brown-black, sharply delimited plaque (diameter, ∼2 cm). Inside the plaque were several dark and slightly elevated papules ( Figure 1 ). The lesion was excised. Histologic sections are shown in Figure 2 through Figure 4 . What is your diagnosis? DIAGNOSIS: Pigmented hidroacanthoma simplex (HS) (intraepidermal acrospiroma). HISTOPATHOLOGIC FINDINGS At low power, the epidermis showed several nests composed of basaloid cells (Figure 2) well defined from the surrounding keratinocytes (Figure 3). These cells were small, uniform, and had much less cytoplasm than the adjacent keratinocytes; the nuclei were also smaller and more darkly stained. Most tumor cells were very pigmented (Figure 4). In the upper dermis