Originally posted by BigDoggProblem
I was thinking Qb5 first and then Bf4+ later, perhaps, but I had missed ...Ra6.
I don't think Ra6 is great for black - 17. ... Kd8 18. c7+ Kxc7 19. Qb5 Ra6 (19. ... Qh2!) 20. Rxa5 Rxa5 21. Bf4+ Kd8 22. Qxa5+ and there's a king hunt. The problem is that black has 19. ... Qh2 and that threatens promotion as well as taking the h2-b8 diagonal which means white has to play 19. Bf4+.
I don't think White can defend against the promotion, after 17. ... Kd8 18. f3 (allowing Bf4) Qh2 black threatens to promote with a discovered check, and after 18. Nc3 Qh2 white loses a whole rook (which rules out the exchange on a5), 18. Kc3 isn't quick enough to stop the promotion, but does allow the f3 idea so white can get out of it for a bishop.
So white's best try after 17. ... Kd8 is probably 18. c7+ Kxc7 19. Qb5 Qh2 20. Nc3 hoping for 20. ... g1=Q (20. ... Ra6!) 21. Rxg1 Qxg1 22. Bf4+ Kd8 Qb6+. Even after 20. ... Ra6 black's king is stuck in the middle of the board and white has some pawns to push.