 After 5.Bf4 Black responds thematically 5...dxc4 (anything else is good for White), and is ready to defend the c-pawn with the standard push b7-b5.White's best attempt might be to continue 6. a4, but Black now has the move 6...Nd5 (since the knight is not pinned by the white bishop as it would be in the standard Bg5 lines).