Captain 'Fun at parties' here: Due to the fact that you give us only 'Doctors' and 'Bob' as possible answers, there is no correct answer to that question, because none of them are lying (assuming that the two doctors are, in fact, female).
So, the only way, that there's a correct answer to that question would be, that Bob or the Doctors are actually lying. And assuming, that Doctors are in general more trustworthy people than random guys named Bob, i would guess that at least one of the doctors is male and Bob is a bloody liar...