We use the indefinite article a/an with any noun (whether singular or plural, count or noncount) when the specific identity of the noun is known to the reader.

«Sherlock Holmes» by Arthur Conan Doyle is a

Where's USB drive I lent you last week?

Ben has terrible headache.

After this tour you have whole afternoon free to explore the city.

«It» by Stephen King is a

Which of the following is NOT a characteristic of a fad?

According to the speaker, what is the main difference between a fad and a trend?

«The Hunger Games» by Suzanne Collins is a

I like blue T-shirt over there better than *the* red one.