Is it a rn or an rn?

The correct phrase is "a RN" when using the acronym for "Registered Nurse." However, it is often pronounced as "an RN" to make the phrase flow more smoothly in speech. The choice depends on whether you're focusing on the initial sound of the acronym (which starts with a vowel sound) or the letter itself. In formal writing, it’s typically more common to use "a RN."