United States' Michael Phelps prepares to swim in the men's 4 X 100-meter medley relay at the Aquatics Centre in the Olympic Park during the 2012 Summer Olympics in London, Saturday, Aug. 4, 2012. Phelps pushed the United States in front to win the medley relay Saturday in the final swimming event of the London Games, after which he is retiring. He leaves the sport with a record 18 golds and 22 medals overall. (AP Photo/Matt Slocum)