The Rule
Use “a” when the following word begins with a consonant
sound; use “an” when the following word begins with a vowel sound . What matters is how the following word is pronounced, not what it looks like. In the example sentence, “MNC” begins with a consonant, but it begins with the sound of the vowel “e,” since we would pronounce it as “em en see.”


Extra Credit
Some people, especially those who are trying extra hard to sound erudite, will use “an” before a word that begins with the letter “h,” such as “an historical artifact.” In the United States, this usage is incorrect. Instead, we write, “a historical artifact.”


Source :https://www.englishrules.com/writing/2005/a-versus-an-the-indefinite-article/..
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