There is a certain common word in English language (say XYZ) such that prefixing it with a letter A creates a new word (AXYZ) that is a plural of another valid English word.

Find it - preferably with pen, paper and your knowledge only.

There is a possibility for several answers...

(In reply to re: Eureka! by Ady TZIDON)

Ady, your initial criticism was correct. The problem does imply that both XYZ and AXYZ are nouns. In English, generally only nouns have plurals, as opposed to some non-English languages where other parts of speech can adopt plural forms. There are exceptions even in English, such as the verb 'are' being a plural form of 'am/is'.
At the minimum, it is given that AXYZ is a plural of XYZ.  AHAS is a noun, but it is not a plural of HAS, though it can be considered a plural of the singular HA, a synonym of AHA. It may even be considered both a plural and a synonym of (XYZ) HAS , but it is not, as the problem specified, a plural of (XYZ) HAS.

As to the proposed word-trick of the two words related via synonyms with AYE being a synonym of YES, and pluralized as AYES, this would be, in my opinion, be acceptable.   

Edited on September 14, 2017, 11:39 pm

Posted by Dej Mar on 2017-09-14 23:08:02