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A 31-year-old man presented with a large pigmented lesion, present since birth on his right upper aspect of his back. On examination, a speckled pigmented plaque measuring 9 × 5 cm was found on the right interscapular area. The plaque was composed of multiple dark-blue macules and papules with intervening areas of faint blue discoloration and brown patches overlapping the blue lesions ( Fig. 1). The lesion was slightly elevated, showing a gradual increase in height from its periphery toward its center. The lesion had enlarged as the patient grew, but there had been no change in color, size, or texture over the past few years. No other pigmentary changes were noted, and there was no family history of pigmentary disorders.

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Figure 1. A large speckled plaque composed of an aggregation of many deep blue macules, papules and small brown patches with intervening areas of faint blue discoloration

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Three skin biopsy specimens were obtained. In the dark-blue papular and macular area, many melanin-laden cells were present in the mid to lower portion of the dermis; these aggregated around blood vessels, the cutaneous nerve, and the pilosebaceous apparatus, and were intimately mixed with irregularly arranged, slightly thickened collagen bundles ( Fig. 2). Many were spindle-shaped, bipolar, dendritic melanocytes, some extending into the subcutaneous tissue. A few melanophages were intermingled with the melanocytes. Some sections contained proliferations of thickened arrector pili muscles. In the brown patches overlapped by the faint blue background area, rete ridges were elongated and the levels of melanin and melanocytes in the epidermal basal layer were slightly increased. Numerous melanocytes were scattered in the mid to lower dermis. These features were consistent with a common blue nevus accompanied by nevus spilus and smooth muscle hyperplasia.

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Figure 2. (A) Clumps of melanocytes intermingled with irregularly arranged collagen bundles in the mid and lower dermis. (B) Smooth muscle hyperplasia around pilar structures in the lower part of the dermis. Hematoxylin and eosin. Original magnification: A, ×40; B, ×400

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Immunohistochemical studies demonstrated a positive reaction of the tumor cells to vimentin and S-100 protein. In addition, many cells reacted to the antibody HMB-45, indicating the presence of premelanosomes. The patient did not wish to undergo surgical treatment, and it was recommended that the condition be monitored regularly to detect promptly any malignant changes.