What about something like this:
"I am unfamiliar with this medication, because as an LPN it is not a medication I can give to you. The RN who will give you this medication will know what it is for, but while you wait I will go and look up the information for you."
This gives an explanation that doesn't make you sound like you don't know what you are doing, is honest, and the patient will appreciate you doing the research and getting back to them.
EDIT: I should add, even if you aren't giving the IV med, it's a good idea to become familiar with all the meds your patient is on so the next time this happens, you at least have an idea