Letter of explaination...

Here is my situation. I am just finishing my second year at a University. They have a two year wait for the nursing program so I am applying to a more competitive university as a transfer student. Now in the transfer application they say that you may include a resume, which I am.
Now backing up a bit, at my current school I have a 3.8 GPA, but I had to withdraw all my classes last semester and failed a class (I retook it and raised my gpa). The reason is because of a serious illness that had to be treated. Now nowhere on the transfer application does it allow for you to explain any failed past classes.

So here is my question: Is it appropriate to include a letter of explaination explaining the withdrawn semester. I could send it in with my resume. Or I can send it in separate. Or I could not send a letter in at all. Unfortunately I have already submitted my application online, so I cannot attach it to my application. I could also possibly see if my advisor will include the letter with my official transcripts when they are sent in, but I doubt they will allow it. What do you think?