Many people from outside of the US and Western Europe receive a vaccine for tuberculosis called Bacille Calmette-Guerin (BCG). It often leaves a pattern of scars on the arm of person who was vaccinated. They look similar (but less distinct in adults) to this:
I’ve suggested to Mika that she should tell people that hers is from a street fight involving bike chains or the time she had to push a child out of the way of a speeding motorcycle and was grazed by the spinning gears.
Thinking about the possibilities makes me wish I had such a scar.