As everyone has said, you only need to give your honest (and brief) explanation for the instances they are asking about. As stated by others, you can pretty much just explain it was medical bill from when you didn't have health insurance, and that now you have it.
Here's what you should know, though! The reason they are asking you for that letter is because the person at the lender who is in charge of your file needs to cover his/her fanny in case your loan ever goes bad someday.
If they are asking you for a letter, it means you're going to get your loan. But they need you to document a reasonable explanation for the bad credit items so that if ever you fail to pay your mortgage, the person who is handling your file right now can show he/she did the proper due diligence on you and he won't get fired. I'm serious.
So just tell them briefly what happened, sign your name, and there you go.