A momentary digression into the irrelevant

Consider it a break from the seriousness and a bit of a challenge.

I've recently been seeing a pop-up ad for some site or another which poses an IQ question:

Which of these does not belong?

U - O - S - E - A

Now I expect the answer they're looking for is "S" since the others are all vowels. But I was wondering how many other answers people could come up with.

For example, "E" because it's the only one not composed of an odd number of lines (1 or 3).

Any more ideas?