PSQR : CFED :: JMKL : ?

The answer is $\boxed{\text{UVXZ}}$.

In the sequence $PSQR:CFED$, each letter is three places after the corresponding letter in the alphabet. So, $P$ is three places after $A$, $S$ is three places after $B$, and so on. This pattern continues for the letters $Q$, $R$, $C$, $F$, $E$, and $D$.

Therefore, the next letter in the sequence is $U$, which is three places after $Z$. The next letter is $V$, which is three places after $U$. The next letter is $X$, which is three places after $V$. The next letter is $Z$, which is three places after $X$.

Therefore, the answer to the question $PSQR:CFED::JMKL:?$ is $\boxed{\text{UVXZ}}$.

Here is a table showing the letters in the sequence $PSQR:CFED$ and the corresponding letters in the alphabet:

Letter | Alphabetical position ------- | -------- $P$ | 16 $S$ | 19 $Q$ | 17 $R$ | 18 $C$ | 3 $F$ | 6 $E$ | 5 $D$ | 4

Here is a table showing the letters in the sequence $UVXZ$ and the corresponding letters in the alphabet:

Letter | Alphabetical position ------- | -------- $U$ | 21 $V$ | 22 $X$ | 24 $Z$ | 25