。
I don't know which doctor she was referring to.
Literal
She [topic-は] which [genitive-の] doctor [object-を] pointed-to [explanatory-question-のか] not-known.
指す here means 'to point at, indicate, refer to' rather than the more common 'to point' or its other senses ('select someone' as in 指名する, 'play a shogi move'). Context disambiguates — the verb is genuinely polysemous. のか分からない is a frequent frame for 'I don't know which/what/whether...' — embedded question + ignorance.