As South, you've opened the bidding one spade and eventually reach a spade contract.

You need to lose only ONE trick. How do you play it?

Lead the queen from dummy. You hope that the queen is covered with the king, crashing the singleton ace. Alternatively, East may play the ace from A-4.

Psychological Error :
Some people lead low toward the dummy, hoping that West holds the K-4 and goes up.

But West will know that you would never lead away from the ace. Whereas the play by East of covering the queen with the king is a legitimate play. Declarer may have ace-fifth and partner holds jack doubleton.