When Raven Kinser walked into a Virginia Department of Motor Vehicles office two summers ago, she checked a box on her driver's license application indicating she did not want to donate her organs in the event of her death, reversing her earlier donor registration. Six months later, after she was declared dead at Riverside Regional Medical Center in Newport News, Virginia, her parents say, they found out that the checked box didn't matter.Raven's case reveals a little-known gap in the U.S. donation system: There is no clear, nationally binding way to opt out — or to ensure a later "no" overrides an earlier "yes" in a different state.This gap, along with a range of other issues related to the organ procurement system, has become a point of bipartisan congressional concern.…