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Isn't she a doctor?
Literal
She [topic-は] doctor [is-not-ではない] [explanatory-のです] [question-か]?
A confirming negative question — the speaker had assumed she was a doctor and is now checking that assumption. ~ではないのですか layers four moves: ではない (negative copula), の (explanatory), です (polite copula), か (question). The combined effect is 'isn't it actually the case that...?' — softer and more thoughtful than a blunt 医者じゃないんですか. The の/んです adds 'as I had been led to believe / as it would seem,' which is the heart of the confirmation reading.