For a patient with acute severe right upper quadrant abdominal pain, the appropriate initial action should be:

For a patient presenting with acute severe right upper quadrant abdominal pain, starting an intravenous (IV) line with normal saline and administering morphine is an appropriate initial action. Administering morphine not only helps to manage severe pain, which can help reduce the patient's distress, but it also plays a crucial role in creating a better environment for assessment and treatment. Pain relief is particularly important because severe abdominal pain can complicate the clinical picture and may lead to involuntary muscle guarding or rigidity, making physical examination more difficult.

Maintaining vascular access with normal saline is important for hydration and can also facilitate the administration of any necessary medications. In cases of acute abdominal pain, especially in the right upper quadrant, it is vital to both alleviate discomfort and prepare for possible further interventions, such as imaging or surgery, depending on the underlying cause.

In contrast, while remaining supine and monitoring oxygen saturation may seem relevant, it does not directly address the severe pain that could indicate a life-threatening condition. Performing a comprehensive abdominal exam is important, but immediate pain management should take precedence for patient comfort and safety. Establishing vascular access and providing a saline bolus alone does not address pain and might hinder effective assessment if the patient is still experiencing significant discomfort.