Black to move
Jan Timman played 5...Ba5.
Anatoly Karpov wrote:
The bishop is badly placed at a5, and the whole of White's subsequent play is built on exploiting this circumstance. I consider that Black should have taken the knight, and then played Nc6...d6...and e5.Karpov won this game and the championship. He had the Black side of this position against Victor Korchnoi in 1993 and played 5...Bxc3.
Anatoly Karpov: Chess is My Life (1980), 28.
Timman's move has been more popular, having been played at least 86 times to 32 for 5...Bxc3. However, White's score against 5...Ba5 has been 67%, while White has scored 56% after 5...Bxc3.
Why is a5 such a poor spot for the bishop?
Karpov elucidates two resulting vulnerabilities in the ensuing comments. One, the presence of the bishop on a5 prevents Black from playing b6, and hence the c5 pawn is difficult to defend. Two, he offers some unplayed lines in which White either wins a piece through tactics, or White's central pawns become mobile at the cost of a pawn.
In the game itself, White's pieces became more active. Under the pressure of an inferior position, Timman blundered away a pawn.