MOSCOW, April 13 (Reuters) - Alexei Navalny, Russia's most prominent opposition politician, is grappling with severe stomach pain in jail that could be some sort of slow acting poison, his spokeswoman said on Thursday. Navalny was suffering from severe stomach pain, she said, and could not eat the prison food provided to him because it was making his pain worse and since Monday has has been banned from buying alternative food. See here for a complete list of exchanges and delays