Because acronyms are read and function as regular words, they are rarely preceded by an article—a, an, or the—in a sentence. However, when they are preceded by an article, choosing a, an, or the depends on how the sentence would sound when read aloud. For example:
“When NATO asked NASA to form a committee, a WHO director became chairperson.”
Because initialisms are read as a string of letters, they often take a, an, or the. For example:
“An MIT graduate was required to take a writing test at the FDA office.”