Final answer:
The best diagnostic test for this patient's condition would be a CT scan, which can construct detailed X-ray images to assess the presence of any abnormalities or pathologies in the abdominal organs. A CT scan would provide a more detailed view of the abdominal anatomy, including the aorta, and can help identify any potential aortic pathology, such as an abdominal aortic aneurysm, which can cause acute back pain.
Step-by-step explanation:
The best diagnostic test to use in the diagnosis of this patient's condition would be a Computed Tomography (CT) scan.
A CT scan can construct detailed X-ray images of the abdominal organs, allowing healthcare providers to visualize and assess the presence of any abnormalities or pathologies. In this case, the patient's physical exam findings, including a palpable infraumbilical pulsatile abdominal mass with left lower quadrant fullness, suggest a potential abdominal pathology, possibly involving the aorta.
A CT scan would provide a more detailed view of the abdominal anatomy, including the aorta, and can help identify any potential aortic pathology, such as an abdominal aortic aneurysm, which can cause acute back pain. Therefore, a CT scan would be the most appropriate test for this patient.