Hey there, I can't hold a medical anymore, but I have MS and before it was clinically definite, I had to do a bunch of MRIs, they gave me Ativan for one of them because it was really an uncomfortable experience. Talk to your AME, explain why you got it, and if you have a good AME it'll be very smooth and easy to take care of. As long as you're not taking it for any legth of time it's not hard to get that handled, and if you are, there's a way to get your medical back.
Not exactly the same for me, but just involve your AME immediately, it's not too painful.
Looks like this question was asked and answered. You are not required to report peri-operative or procedural meds, if so as an extreme example you would be required to report sedation for a colonoscopy
That being said if you were to report it I would just add it to the discussion about the doctor visit for the MRI, something like "MRI for xxxx, valium given prior to procedure once." Won't be an issue.