Analogies must start with scr, spr, str, thr

5 letters
broom : sweep :: brush :

The purpose of a BROOM is to SWEEP.

The purpose of a BRUSH is to SCRU__?

To find the answer to this analogy, we need to focus on the relationship between the words "broom" and "sweep" and then find another set of words with a similar relationship.

In this case, "broom" is an object that is used to perform the action of "sweeping." So, the relationship can be described as the tool or object used to carry out a specific action.

Now, we need to find another set of words that follow the same pattern with the given letter pattern "scr," "spr," "str," or "thr" and also have a similar relationship.

Taking into consideration the letter pattern and the desired word length of 5 letters, one possible answer could be:
- Scrub : clean

In this analogy, "scrub" is a word with the desired letter pattern and also represents an action similar to "sweeping" in the previous analogy. It is a verb used to perform the action of "cleaning." So, the analogy "broom : sweep :: brush : scrub" can be formed.