、。
She's not a nurse — she's a doctor.
Literal
She [topic-は] nurse [not-ではなく], doctor is.
ではなく is the negative copula in continuative form: 'is not X, [but rather]' — used to negate one option and pivot to the correct one. The full pattern XではなくYだ ('not X, but Y') is the standard Japanese way of correcting an assumption. The form ではなく (rather than plain ではない) sits in conjunctive position, leaving the sentence open for a continuation; in writing you'll often see this with no comma, but the comma here makes the contrast clearer. Note that 看護婦 vs 医者 also evokes a real-world hierarchy issue — being mistaken for a nurse when one is a doctor is a complaint female physicians often raise about patient assumptions.