Hello, I am currently a 4th year medical student and I interview for the admission committee at my school. I have also taken medical ethics. A lot of the pre-med responders have replied with thoughtful replies, but lack understanding of the complexities of clinical medical ethics.
First off, asking ethical questions do have some value, but it shows a lack of interviewer experience or forethought to ask about healthcare ethical dilemmas. Applicants usually have a superficial understanding of medical ethics, which is more complicated than your every-day ethical issues.
Regardless, the best way to handle these questions is to always respond with whatever has the patient's best interest at heart. For your specific example: It is wrong for the surgeon to refuse the procedure simply because the patient is HIV+ (it's actually illegal in many states to refuse care simply because a patient has an infectious disease). However, if the surgeon is concerned that the risk to the patient is too great with the procedure, then he can refuse to perform the operation (i.e propensity for blood clots, great risk for post op infection, severe anesthesia reaction, etc. and by the way patients with well controlled HIV are not at increased risk for post-op infections.). Its all a matter of weighing benefits (improved mobility or quality of life) against the risks.
Also, using the "its against your religion point of view can actually backfire". Example, in most states if your patient comes to you and asks for something that goes against your religious beliefs (morning after pill has recently been a hot topic), you are allowed to refuse this service, but you are REQUIRED BY LAW to refer them to someone who is able to provide the service.
Again, the take home point is to DO WHATEVER IS IN THE BEST INTEREST OF THE PATIENT, not what is in the best interest of the doctor.
I hope this helps.