It doesn't make any difference at all whether the article is modifying an acronym, an initialism, a proper noun, a French borrowing, or anything else. English article form is determined solely and entirely by pronunciation. And not at all by spelling.

The rule for the pronunciation of articles in English -- definite and indefinite -- is that they have one form before consonants (note, real consonants -- sounds -- not "letters" in a writing system), and a different form before vowels (ditto).

Hence, how you say it is what counts. Nothing else does.

• Before vowels -- Indefinite an /ən/ and Definite the /ði/:
  an hour, an SOS, an A-to-Z selection, an EE degree, an idiot
  the hour, the SOS, the A-to-Z selection, the EE degree, the idiot (all pronounced /ði/)

• Before consonants -- Indefinite a /ə/ and Definite the /ðə/:
  a URL, a snafu, a Charlie Foxtrot, a moron
  the URL, the snafu, the Charlie Foxtrot, the moron (all pronounced /ðə/)

Most native English speakers never notice that there are two different pronunciations for the, but non-native English speakers need to know this immediately.

It doesn't make any difference at all whether the article is modifying an acronym, an initialism, a proper noun, a French borrowing, or anything else. English article form is determined solely and entirely by pronunciation. And not at all by spelling.

The rule for the pronunciation of articles in English -- definite and indefinite -- is that they have one form before consonants (note, real consonants -- sounds -- not "letters" in a writing system), and a different form before vowels (note, ditto).

Hence, how you say it is what counts. Nothing else does.

• Before vowels -- Indefinite an /ən/ and Definite the /ði/:
  an hour, an SOS, an A-to-Z selection, an EE degree, an idiot
  the hour, the SOS, the A-to-Z selection, the EE degree, the idiot (all pronounced /ði/)

• Before consonants -- Indefinite a /ə/ and Definite the /ðə/:
  a URL, a snafu, a Charlie Foxtrot, a moron
  the URL, the snafu, the Charlie Foxtrot, the moron (all pronounced /ðə/)

Most native English speakers never notice that there are two different pronunciations for the, but non-native English speakers need to know this immediately.