Exon is one of those wonderful wacky genetic terms. I’ll see if I can break things down.
You know that a gene has a particular DNA sequence that codes for the production of a protein or an enzyme (such as 21 hydroxylase) An exon is the region of that gene that actually translates into the protein/enzyme associated with that gene. However, there are also regions called introns between the exons that do not translate into the protein/enzyme.
So in the case of the 21 hydroxylase gene, the gene is kinda set up like this:

Exon 1--intron 1--exon 2--intron 2--exon 3--intron 3--exon 4--intron 4

and so on and so on up to exon 10. Your body then makes something called RNA, which is the DNA sequence of the gene of interest without the introns, so you get something like this:

exon 1--exon 2--exon 3--exon 4--exon 5... and so on

This RNA sequence is what is actually translated into your 21 hydroxylase (or whatever else the gene is coded for)
Just remember that the exons are the important part in terms coding for 21 hydroxylase production, and introns are kinda extra DNA sequence in between your exons. Geneticists just like number these things so it gives them a better idea of what is going on where!
I would think that they would be able to figure out if your daughter’s mutations occur on the same gene or not with a closer look at her DNA- I don’t see why they couldn’t sequence it or something like that.
I hope this helped- and I hope the docs can figure out where her mutations are!

Martha