Student or Learner
My wife received a notice for "an" MRI. Is this because "M" is pronounced "em"?
Yes, the important thing is the sound of the name of the letter you are reading out. So:
An H (I'm really hoping no-one is going to try to tell you it's pronounced "haitch" and should be preceded by "a"!)
Remember - if you don't use correct capitalisation, punctuation and spacing, anything you write will be incorrect.