AMA Manual of Style - Stacy L. Christiansen, Cheryl Iverson 2020

Articles
Grammar

There are 2 types of articles—definite and indefinite—that function as adjectives and precede a noun or noun phrase. Which type to use depends on the context.

7.3.1 Definite Articles.

The definite article the describes a specific object. It can be used to describe both singular and plural nouns as well as both common and proper nouns (note that sometimes the is part of a proper name).

The paper has been accepted.

The journals are organized by issue number.

Have you read the research reports in The Lancet this week?

7.3.2 Indefinite Articles.

The indefinite articles a and an indicate a nonspecific object. Their use is exclusively for singular items; nonspecific plural nouns drop the article.

A paper has been accepted.

Have you read an issue of the journal this week?

Journals are organized by issue number.

7.3.2.1 Selecting a or an.

Deciding whether to use a or an depends on how the subsequent noun (or modifier) is pronounced aloud, regardless of spelling. “An” is always used before a vowel sound (but not necessarily always before an actual vowel) (see 11.11, Articles).