A 54-year-old man selfrefers to dermatology for evaluation of a very itchy rash that manifested abruptly about three weeks ago. He says there is “no apparent reason” for it, although he acknowledges that he has been “really stressed” lately.

He has several other health problems, most of which are musculoskeletal (including back pain and recent shoulder surgery). He has not been out of the country, has no pets, and lives alone. He denies ever having hepatitis C.

The rash is confined to his low back, running from about T-6 to just above the sacrum. It is composed of discrete, purplish brown, planar papules ranging from 2 mm to just over a centimeter. The lesions—which number about 50—are mostly polygonal (angled margins, not round or oval) and barely (if at all) palpable. The surfaces of most lesions have a shiny, frosted appearance.

The patient’s nails appear normal. There are no other areas of involvement (eg, elbows, knees, scalp, genitals, or oral mucosae).

A shave biopsy is performed on one lesion. The results show obliteration of the dermoepidermal junction by a lymphocytic infiltrate, resulting in an irregular sawtooth pattern.