A 10-month-old white boy presented with a 5-month history of an asymptomatic rash on his lower back. The lesion was not present at birth and the neonatal check was unremarkable. The patient was born at full term via normal vaginal delivery after an uncomplicated pregnancy. He had since met all his developmental milestones and was otherwise well. His medical history included transient infantile seborrhoeic dermatitis of the scalp and temporary failure to thrive secondary to gastro-oesophageal reflux disease, effectively managed with omeprazole. Mild eczema was present in the family history.Physical examination revealed a mildly depressed atrophic patch, 120 9 50 mm in size, with a well demarcated cliff-drop border, on the patient's lower back (Fig. 1a). The lesion crossed the mid-line, and subcutaneous blood vessels were distinctly visible. There was no induration, scarring or lilac ring border. Spinal ultrasonography scan was normal.
Histopathological findingsA 6-mm punch biopsy was taken from the lesion border, including areas of normal and atrophic skin for comparison. Figures 2-5 show the histology of the normal skin on the left, while on the right is the corresponding atrophic lesion showing dermal and epidermal atrophy with increased fibroblasts. No inflammation or features of scleroderma were present. Elastic stain showed reduction of elastin (Fig. 5). Staining with Hale colloidal iron for mucin was unremarkable, and S100 staining was negative. Periodic-acid-Schiff did not reveal increased glycogen or any organisms.What is your diagnosis? Figure 1 (a) Well-demarcated atrophic lesion on lower back crossing the mid-line at 10 months of age; (b) at 20 months of age, the size and border of the lesion had remained stable in proportion to the patient's body.