Should you use ‘a’ or ‘an’?

Determining whether to use “a” versus “an” should not be confusing, but it
is.
This week, I had a prolonged discussion with a co-worker about why “an MRI”
is correct and “a MRI” is not.
It turns out that many of us were taught the wrong rules for use of these
indefinite articles. I remember being told to use “an” when the word
preceding it starts with a vowel and to use “a” when the word preceding it
starts with a consonant