What's Black's best move?

What's Black's best move?


This problem is inspired by an unplayed alternative line from a 3/2 blitz game I just played.  (I tweaked the arrangement of the pieces to accentuate the key move.) 

I picked this problem for my blog because it appears confusing, at least to my amateur eye.

For the moment material is equal. Each side holds 19 points in material. 

Each side has positional advantages and disadvantages.

White enjoys the rook pair while Black holds the bishop pair.

White has two pawn islands while Black's pawns are scattered across the board.

It's clear, however, that Black is about to fall behind in material. Although White's knight is poorly placed on the edge of the board, Black's knight is en prise. Moreover, White's rook on d1 threatens Black's unsupported bishop on d6. 

It's also clear that Black cannot save both its knight and bishop. If Black plays 1...Nb5, attempting to save its knight while supporting its bishop, then White can play 2.a4, once again putting Black to a difficult choice. Similarly, if Black plays 1...Nf5, then White can play 2.g4 to the same effect.

Black could choose to save its bishop over its knight by playing 1...Be5 but once White plays 2.cxd4, capturing a knight with a pawn, it's tough to see how Black can find any advantage and might be behind in practical play.

Black could also push a pawn with 1...e5 but that will leave Black behind in material.

Of course, there're other possible moves but none of them seems to resolve the key challenge confronting Black.

Yet there is a single move that affords Black a decisive advantage, one that should be sufficient to win the game.

What is Black's best move?

And more importantly why is it the best move?

I look forward to your comments.