You are on the right track....
A medical diagnosis basically describes the reason the pt was admitted, a disease process, mechanism of injury, or even a generalized complaint. So, there is a reason the pt had hip surgery. Doctors can admit a pt and put a procedure as the medical diagnosis, but try and see if you can find why the pt had surgery. So, you are right that hip surgery isn't a definitive diagnosis, but that ok if the doc wants to go with that. Part of it also has to due with billing. They can bill out for surgery if that's the reason for admission. But, there is a reason for the surgery.
No, it was the "related to" part of of the nursing dx. The instructor marked it wrong and said not to use medical diagnoses but to use the definition. Okay so the definition of left hip surgery is what? Surgery of the left hip? I'm just tired. Nursing school is kicking my butt. Two months left and I'm done!! (graduated).
She may have meant, the definition of the nursing diagnosis, or the defining characteristics of the diagnosis you chose. I would clarify that with her. But read on.
A nursing diagnosis statement translated into regular English goes something like this: "I think my patient has ____(diagnosis)_____________ . He has this because he has ___(related factor(s))__. I know this because I see/assessed/found in the chart (as evidenced by) __(defining characteristics)________________."
"Related to" means "caused by," not something else.
To make a nursing diagnosis, you must be able to demonstrate at least one "defining characteristic." Defining characteristics for all approved nursing diagnoses are found in the NANDA-I 2012-2014 (current edition).
It is perfectly acceptable (and you will find it in NANDA-I) to have a medical condition (like "intramedullary nail L femur on Date") as a related factor. You can look it up and show it to your instructor.