It’s a uvula. The rule for using “a” or “an” is based on pronunciation—not spelling. If the following word has a consonant sound, then the indefinite article to use is “a”.
It’s a user; thus, we say “a user-friendly”, “a user interface”, “a user fee”, etc. The word “user” begins with a consonant sound, a “you” sound, and therefore takes the indefinite article “a”.
It’s a useful... The rule for using “a” or “an” is based on pronunciation, not spelling. Since the word useful starts with a consonant sound (a “you” sound), it takes the indefinite article “a”.
It’s a Uruguayan. The rule for using “a” or “an” is based on pronunciation, not spelling. The vowel “u” in this case makes a consonant sound (a “you” sound), and therefore takes the indefinite article “a”.
It’s a university. The initial letter of the word “university” makes a consonant sound (a “you” sound); consequently, it takes the indefinite article “a”.