I’ve blogged before about when to use “a” and “an” with initialisms. Here’s a real-world example, taken from Huddleston and Pullum’s A Student’s Introduction to English Grammar:

It is typical for the subject of a clause to be an NP.

“But Karen, ‘n’ would take ‘a’ because it’s a consonant!”

Nope. “N” takes “an” when it’s pronounced as itself, the letter “en.” It begins with a vowel sound, which takes “an.”

Clearly, the authors intend for us to say “en pee” rather than “noun phrase.” The indefinite article “an” is the cue.