It's very unclear. Probably 4.Ne5 d5!? (Qe7?!) ∞
An easy and safe way to respond to Ruy Lopez

The line 3...Nf6 4.0-0 Bc5 have never seen it.
What's wrong with 3...Nd4 Pfren?
3...Nd4 4.Nxd4 exd4 5.Bc4 (this is the modern handling of the Bird) which prevents the thematic ...Bc5 due to Bxf7+/ Qh5+. It's hard for Black to achieve equality in this line. There is Carlsen's emphatic loss to Saric, and a few newer outings which went badly for Black.

A bit off topic, but I was looking at the rook ending played by Caruana.
I would have immediately played 29. Rxa3.
Then I guess the problem would have been ...Rc5 and black seems to get a passer.
pfren, do you have any book to suggest for this kind of rook endings?
I know all the famous positions (Lucena, Philidor, etc.) but this practical tricks of rook endings I can't find easily in the books I have.

plutonia, there's a very good little book by Nikolai Minev. Actually it's a trimmed down version of the Bulgarian original.
http://www.amazon.com/A-Practical-Guide-Rook-Endgames/dp/1888690224
There are others too, of course, but I do like this one, and use it at my lessons.

plutonia, there's a very good little book by Nikolai Minev. Actually it's a trimmed down version of the Bulgarian original.
http://www.amazon.com/A-Practical-Guide-Rook-Endgames/dp/1888690224
There are others too, of course, but I do like this one, and use it at my lessons.
Thank you very much, I appreciate :)

http://exeterchessclub.org.uk/content/rook-endings-theory
if that one example does not help, there's no need for a book either
The problem with 3...Nf6 4.0-0 Bc5 is 5.Nxe5! which is slightly better for white.
You can use the 3...Bc5 move order to get in the same position:
4.0-0 Nd4! is OK for Black, but not necessarily dull- e.g. white may try a strange piece sac ala- Cochrane gambit, which is actually better than the original: 5.Nxe5!? Nxb5 6.Nxf7 Kxf7 7.Qh5+ g6 8.Qxc5 c6 9.c4 etc. Black should be more than OK, but white has his own chances.
4.c3 Nf6!? 5.0-0 0-0.
White can play 5.d4 exd4 6.e5, but I think Black is OK after 6...Nd5 7.0-0 Be7!
Just do some homework, the variation is rather rare.
LotsofSnuff227: There is no Karpov- Kupreichik game where all those blunders were played. In the actual game Karpov played 6.d3, and won in fine style (typical Karpovian).