Baltimore Ravens offensive lineman John Urschel has quit the NFL just days after the world's biggest study into a degenerative brain disease showed that it affects 99 per cent of players. The study, which was announced on Tuesday, found that out 111 NFL players' brains examined by Boston University, 110 showed signs of damage due to chronic traumatic encephalopathy (CTE). That news so shook Urschel - who is studying for a mathematics PhD at MIT in the off-season - that he quit on Thursday, a friend told ESPN. Urschel, who is 26, is studying spectral graph theory, numerical linear algebra...