Answer:
Pathological Fracture of the femur.
Step-by-step explanation:
Okay, let us do the analysis by checking out some of the Important facts about this question. So, in the question we the woman whose name is Grace is 81 years old, the woman also suffered from dementia(things like forgetfulness). This same woman has fracture of the femur(which is the first reason she was admitted) and metastatic bone cancer because of uterine cancer.
With all the information above we can conclude that the woman's principal diagnosis is Pathological Fracture of the femur.
We can also see this in the Second statement of the first paragraph in the question; ''was admitted due to a fracture of the femur, pathological in nature". That is is to say the reason she was admitted initially was because of Pathological Fracture of the femur.