InfoPath Dev

I have a pre-existing schema that I am not able to touch so I need to use a secondary data source for certain information. I need to populate descriptions for certain fields dependant on a selection from a drop down menu. I want all these descriptions stored in xml docs so they can be changed very easily. I want to know if there is a way to do this using infopath, because I have not come across anything that could solve this problem.

Ok, I figured out how to grab the data from the xml source using the expression box. Now the only problem I will have is using a specific xml file which is dependant on a user selection.

 User selects option A from drop down list

 Form loads rules specific to option A

I want to do it this way because I don't want to have to use conditional formatting and have a million expression boxes on my forms when I am designing them.