Nomad by George O. Smith

Guy Maynard faced the President of the Court, who said to him: "Maynard, your story is absurd. That you spent a year on an unknown planet sounds impossible. But -- there is one bit of evidence which, if you can explain, will be discarded. Early medical records claim that you have a MacMillan burn beneath your right arm. It is further stated that if this scar is not removed, it will turn into cancer. No record can be found of its removal -- yet it is gone. To clear yourself, name the surgeon that removed the dangerous scar."

Maynard blinked. He'd forgotten the scar entirely. It had been a minute speck that had never given him a bit of trouble.