It would be tempting to play the simple QxN for free here—and erroneous. Don’t play any capture, not even of a loose piece, until it’s clear that you can’t do better. Among other things this means examining checks you can give and their consequences. White has a few with his queen; the most interesting is Qe8. Black’s king is forced to a7 (the White knight seals off b7)—and now rethink the board and ask if new tactical strikes might have become possible. With your knight so prominently posted it is natural to look for forks it might inflict, which train of thought leads to Nb5+. White wins the queen for a knight.