Put a calculation like this in your validation dialog:
PatternCount ( Field A ; "X" ) < 1 ;
Self <= 99 ;
Note that this will validate any time there's an "X" in the field, not just when it's a "word". What do your values actually look like? Any occurrences where "X" is not a standalone word?
The "X" just represents the value of the drop-down list for which the condition should apply. Part of the text of this field can be identified as a unique pattern, so no problem.