Which article (an/the) is correct in this context?
This is an exercise we had to complete in our grammar class. I filled it up like this:
Write the missing articles, if one is needed.
A: Who is the man talking to C?
B: I'm not sure. I think he is a doctor.
A: He looks familiar. Do you know where he works?
B: I think he was the doctor who looked after C when she was in __ (zero article) hospital.
A: No—he is the assistant nurse.
My teacher said that all of it is correct, with only one exception. He says that it should have been an instead of the in the last blank. His reasoning is that ‘assistant nurse’ is being mentioned for the first time. And there are thousands of assistant nurses; we're talking about no one in particular.
But, I think that a particular ‘assistant nurse’ is in question—the one who looked after C when she was in hospital.
So which one of us is actually correct?