I'd never heard of Entre Rios, and after reading up on it, I'm not sure it belongs as an independent country. Still, I'm glad it was here so I could learn something.
Have to say I agree with Eutychus. An unrecognized, unstable breakaway polity doesn't really count as a country. The "countries of the world" quiz doesn't include rebel groups with territory or unrecognized attempted secessions.
"Entre Ríos" region is also called "Mesopotamia" in Brazilian History books, both terms meaning "between rivers" (in Spanish and Greek, respectively). I think this could be consiedered too.
Great quiz!