Bidding commentary:3 is a splinter jump. It describes a game-forcing spade raise with a singleton diamond. South’s 4 is a cuebid and 4NT is Roman Key Card Blackwood, the trump king counting as one of the five key cards. The 5 response shows one key card (the K), and North bids slam expecting South to have both black kings.
Defensive commentary: A diamond lead would have defeated the contract, but a heart lead looks normal
Play commentary: It looks for all the world as if there is a diamond and a heart loser, but there is a chance! Declarer must hope that the defender with the A has fewer than three hearts.
Declarer wins the opening lead, draws trump, plays four rounds of clubs, cashes a second heart (key play) and exits with a diamond, an equal-length suit. Sure enough, East, the player with the A, started with two hearts and is forced to lead a diamond, allowing West to discard his remaining heart while ruffing in dummy.
Holding the A-K-x-(x)-(x) opposite x-x-x in a side suit in a trump contract does not necessarily mean a loser! If you can eliminate one or more suits and cash a couple of tricks in the suit you’re worried about, you can throw an opponent in and, if you’re lucky, force him to give you a ruff-sluff for your contract. Something to think about.