Source: http://cdn.acbl.org/nabc/2018/02/bulletins/db4.pdf

Dealer South. Both Vul

10 5 4
9 2
A K Q
K J 7 5 3
7 6
K Q 5 4
9 7 4
10 9 6 2
9 8 2
A 10 8 3
10 8 6 3 2
A
A K Q J 3
J 7 6
J 5
Q 8 4
West North East South
1
Pass 2 Pass 2
Pass 4 End

Opening lead: K

Bidding commentary: As South, treat a solid five-card major suit as a six-card suit and bid accordingly.

Eddie Kantar
Eddie Kantar

Defensive commentary: As East, avoid the common defensive error of signaling interest or disinterest in the suit partner has led (K) before looking at the rest of the hand.

Here you have the blank A and would like a club ruff. You can get it but you must overtake the K, cash the A, exit a heart to partner’s queen, and ruff the club return.  Down one.

If you look only at hearts, you are apt to signal encouragement to show the ace. Once partner continues a heart, goodbye club ruff, goodbye defeating 4.

The lesson to be learned here is that third hand should take a moment to get an overview of the deal, not just the suit partner has led, before playing to the first trick.