What's this painless rash overlying a scar?

A 54-year-old woman presents as a new patient with a one-year history of worsening painless erythema overlying a scar on her dorsal left wrist.

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Professor Dedee Murrell is head of the department of dermatology at St George Hospital, Sydney, and conjoint professor at the University of NSW.

This article was co-authored by Dr Timothy Cowan, a dermatology registrar at St George Hospital.