What was black's last move? Hadta have been ...f5. Therefore en passant should work for a mate.
Mate in... what?
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Oh wait, something may have been captured on h6...
If it was ...f5, you would still need to find out if the pawn was on f6 or f7 before.
For the black king to get to b1, it would have to come along the a rank, it couldn't come through the c2 hole. Therefore the white king must have moved. Castling is impossible for white.
OK, so what about en passant?
If the black pawn has just moved to f5 from f7, then the white bishop on e8 must be a promoted pawn which came by the way of e7. Not sure what it leads to...
I've posted this on the main chess.com forum, but I figured I'd port it over here too. What is the smallest number of moves White can mate in? Of course, it's really a question of whether White can castle or en passant to mate in one. The board is oriented with a1 at the bottom left.