Her brother used to work as _______ MC on many TV talk shows.
Before a noun indicating a profession, when it is mentioned for the first time => use “a” or “an”.
a + noun whose pronunciation begins with a consonant
an + noun whose pronunciation begins with a vowel
MC /ˌem ˈsiː/ has a pronunciation starting with a vowel /e/ => use “an”.
Her brother used to be the MC for many talk shows on television.