Here’s your hand as West and dummy’s which is your LHO:
West North East South
1D P 1H
P 1S P 2C
P 2D P 2N
P 3N P P
You lead the club 3, partner plays the king and declarer plays low. Partner returns the club 4 and declarer takes the Ace. Declarer leads a low heart to the ace and a low diamond back to her King, and follows with the 3 of diamonds, losing the finesse to your partner’s Queen and partner returns the Jack of Clubs which you overtake with your Queen and take the 9 as the ten fell from declarer’s hand on the lead of the jack. Now, here’s what’s facing you:
You can lead either a spade or a heart. You’ve got your book, so all you need is one more trick to set the contract. What do you lead? You look at the board and realize that all four diamonds are good. If declarer has the king of spades, the hand is over if you lead a spade. But who has the king of spades? Maybe declarer has both. If so, the hand is over anyway. But if declarer has the king of hearts and not the king of spades, you can only set the contract if you lead a spade.
You shouldn’t be tempted to lead through the void because declarer’s first call was hearts and her second call of 2C should promise five hearts. Also, if she bid a five card suit, you can clearly see that she lacks the AQT. Did she bid a five card suit headed by only the J empty fifth? While that’s not impossible, it’s implausible. Further, if she lacks both the king and the queen, would she immediately lead to the singleton ace of hearts? Unlikely because that would give you two heart tricks if you got in.
Another factor mitigating against your leading hearts is that you are undoubtedly giving her two heart tricks because you will be leading into her KJ from your queen. Since you have the queen, that gives her two tricks to take and sluff her two losing spades on. The facts that she bid hearts, that you have the queen ten, and that she immediately led to the singleton Ace all mitigate towards the conclusion that she has the king of hearts and that your partner could have the king of spades.
In the actual hand, you led a heart from your queen-ten, giving declarer two heart tricks so she could sluff the two spade losers and make the contract. Here’s the four hand layout:
? T852 ? KJ4
? QT54 ? 973
? 5 ? Q87
? Q953 ? KJ73
By leading a spade, declarer either has to finesse immediately and lose the king, or go up with the ace, run the diamonds and then lose the last two tricks. But even if she finesses and loses the king, a savvy East will take the king and immediately return a diamond, putting her back on the board so she’ll still lose two spade tricks and go down 2. A heart lead makes the contract.
This isn’t even a thinker. You must lead a spade. If declarer has the king, then she makes the contract. The odds that she lacks the king of hearts (along with the Ace, Queen, and Ten), the suit she bid, are slim and none. So you have little to lose by leading the spade.