When my oldest daughter showed me this incriminating mark on her arm two in a half years ago, I thought, "Who did this to her?!" She told me she had just noticed it, and had no recollection of even getting hit on the arm.
When it didn't go away after a couple of weeks, I assumed it was just a strange rash, or other equally weird or rare, harmless thing that my side of the family seems to be prone to. If nothing else, it was something rather striking to me, especially when it becomes a deeper purple that made my daughter even more unique and precious to me. Still, every doctor she saw, I asked. They all were baffled.
Finally, we saw a dermatologist. He instantly and confidently told us it was a type of port wine stain birthmark. These vascular birthmarks are normally very noticable at birth. Hers might have been very light, he told us, escaping detection--ironically including that of her very watchful pediatrician whom she saw as an infant who had a large port wine stain, herself!
We weren't worried, but it's good to finally know what it is! --LKR