Source: ACBL Bulletins
Bidding commentary: Looks sane
Opening lead: Also looks sane
Play commentary: After winning the spade in dummy, lead a low heart, protecting against all four hearts on your left and also giving you a chance to finesse the 10 if the king loses to the ace. You probably would not take that finesse, however, because a 2-2 division of the hearts is more likely than West’s having A-J-x.
Defensive commentary: After winning the 
A, West can see further defensive tricks will come from the minor suits – and he needs three tricks to defeat the contract. There are two possibilities: (1) Partner has the 
A-Q and that
plus your 
K will give you the three needed tricks; (2) Partner has the A, in which case you can switch to the K and score two tricks in diamonds followed by a diamond ruff. Which shall it be?
If partner has the A Q, declarer has jumped to 4 after partner’s single raise with 7-8 HCP: the 
A, the K and maybe the J. Hardly. Actually, if partner has the A Q, you can exit with a spade or a heart and still beat the contract by shifting to a club when in with the K.
If partner just has the A, declarer has the A, the and the A, plus possibly the J or Q. That is a much more likely hand considering the bidding. It’s clear your best chance is in the diamond suit, so shift to the king at trick three. If you are right, it will be the only card in your hand you can lead to defeat the contract and be partner’s hero… at least for this deal.
Tip: As a defender, go back to the bidding. It often tells you how many points declarer has, which helps you plan the defense. Further, if you know how many points partner has, you will know how many points declarer has.