I knew I’d written about this before. Here’s the proof. However, I’ll write about it again because it keeps coming back.

The issue at hand is whether one uses a or an before a given abbreviation. I’m sure that you were drilled on this in school (I sure was) by a teacher who insisted that you use a before a consonant and an before a vowel.

That’s partly correct.

See, it’s not about the letter itself. It’s about the sound. It’s about how we pronounce the abbreviation (acronym, initialism, whatever). Does the pronunciation begin with a vowel sound or a consonant sound?

Let’s take two common medical abbreviations that begin with the letter M, and see what happens when we use an article with each.

First, let’s say you have recurring migraines. Your neurologist is likely to schedule you for ___ MRI. Do you use a or an there? If you’re like most of us, you call that test an EM-ARE-EYE. It begins with a vowel sound, E. So, you would use an with it: You’re having an MRI.

Now, let’s say that you’ve been hospitalized and, heaven forbid, you’ve gotten a staph infection. And not just any staph infection, but one of the really nasty resistant ones. You have ___ MRSA infection. A or an, there? Again, most of us would just speak it as if it were a word (MERSA). That means it begins with a consonant sound, M-. The article a goes with consonant sounds: You have a MRSA infection.

Similarly, you may be visited by an FBI agent, but you pay a FICA tax. If you’re a medieval recreationist, you may be an SCA member. (Perhaps you prefer “a SCAdian.” Lady Grietje Crynes, former seneschal of the now-dissolved Shire of Rockwall, Kingdom of Northshield, at your service.)

It’s all about that sound. A goes with consonant sounds. An goes with vowel sounds.