Final answer:
Physicians have an ethical and legal duty to treat all patients in need, particularly in emergency situations as mandated by the EMTALA, but can generally choose whom to treat in non-emergency settings without discriminating.
Step-by-step explanation:
The default starting point for a physician's duty to treat is A) To treat all patients in need. This ethical obligation is rooted in the principle of medical beneficence which implies that physicians have a professional duty to act in the best interests of their patients. This duty is also legally supported in emergency situations under the Emergency Medical Treatment and Active Labor Act (EMTALA), which requires hospitals to provide care to anyone needing emergency healthcare treatment regardless of citizenship, legal status, or ability to pay. However, outside of emergencies, physicians may choose whom they will treat, as long as they do not violate laws against discrimination.