Abstract
A 4 month-old infant presented with a brownish, hairy plaque on the posterior site of his left shoulder. A biopsy specimen showed a slight elongation of rete ridges with hypermelanosis in the basal layer and an increased number of well-defined, smooth muscle bundles throughout the whole dermis. The presence of smooth muscle fibers was confirmed by Azan-Mallory staining. The smooth muscle bundles were positively stained with desmin but negative for vimentin by the PAP method. These results indicate that the smooth muscle bundles in the present case showed a well differentiation comparable to that of the arrector pili muscle. A similar eruption was also later recognized on the right thigh of his younger brother.
