CASEA 16-year-old boy came to the dermatology clinic seeking evaluation of a lesion on his knee. The boy's parents had first noticed skin changes in the area about 2 years before, when the boy was 14 and just entering puberty. The lesion began as focal darkening of the skin on his left knee. This was followed over the next months by the appearance of an increasing number of hairs within the lesion, which continued to enlarge. Alarmed at the lesion's changing appearance and increase in size, the boy's parents consulted his pediatrician who, in turn, referred the family to dermatology.
The boy was in good health. A uniformly pigmented, 20 25-cm, light brown macule with irregular margins (see Figure 1) covered the entire surface of the left knee, spilling over onto the anterior thigh and leg; the posterior leg was spared. Compared with the surrounding area, the number of hairs within the borders of the lesion was definitely greater. No other skin changes were visible on his body.
A punch biopsy was performed. The specimen was embedded in paraffin to facilitate the cutting of ultrathin sections, which were then placed on slides and exposed to hematoxylin-eosin stain. This so-called differential staining imparts either a blue or pinkish orange color to the various tissues, allowing the viewer to visually distinguish, for example, nuclear tissue (stains blue) from fibers of the dermis (stain pinkish orange). This patient's specimen exhibited elongated rete ridges, an increase in bundles of smooth muscle fibers unassociated with either follicular units or vasculature, and increased melanin in the epidermis.
Click NEXT to find the true diagnosis.