Ned is a 14-year-old who presents with concerns about unsightly horizontal marks on his lower back for the past two months. 

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Ned is a 14-year-old who presents with concerns about unsightly horizontal marks on his lower back for the past two months. He finds these very embarrassing when he swims.

Ned is otherwise well, with no past or family history of note, and takes no regular medications.

His height is 30th centile, weight 10th centile and BMI 20th centile for age, which is consistent with historical growth. Vital signs are normal and he appears generally well.

His skin and joint examinations are normal, apart from multiple horizontal, linear, erythematous-to-purpuric marks, on the lumbosacral region (see figure A) and wrinkled, atrophic, faint white lines on the knees (arrows, B).

He is Tanner stage 3-4 and has no endocrinological findings of note.