Text is
Is this a pen? - No, it is not a pen.
Why do we use indefinite article in the second case instead of definite article before pen?
Person 1: Do you want an apple?
Person 2: Yes, I want an apple.
Person 1: OK, you can have the apple that is on the table, not the one on the couch.
"the" is specific. Wanting a thing or not wanting a thing is not specific. It is only afterwards that a thing can become or does become the thing.
He has a large freckle on his nose. The freckle has been there for ages.