Unexplained bruises - any causes that aren't really bad news?

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I friend's teenage daughter has been developing bruises all over her body.

She went to doctor who sent her for blood tests but later called her mum and said, "I'm worried about DD, please take her for those tests urgently". That was a couple of days ago and they're going back for the results tomorrow. I know my friend is very worried and I can only think of awful things (and I'm not going to Google). Obviously I haven't voiced any of these fears to my friend. Are there any harmless reasons for unexplained bruising?

low iron

My cause was severe anaemia. Unpleasant, but readily treatable.

I suffered from unexplained bruising during my teenage years and into my 20's. I did have a slight discrepancy in one of my haematology blood tests but the bruising seemed to settle down and that discrepancy is now viewed as insignificant.

My hyper mobility has caused them.

Also pregnancy, but that's probably my body being weird.