A patient presents with severe left-sided back pain and hypotension. What is the most likely diagnosis?

The presentation of severe left-sided back pain combined with hypotension strongly suggests the possibility of an aortic aneurysm rupture. In this scenario, the sudden onset of severe back pain can originate from the area where the aneurysm is located, particularly if it is a thoracic or abdominal aortic aneurysm. The hypotension arises when the aneurysm ruptures, leading to internal bleeding, which results in a rapid decrease in blood volume and blood pressure. This combination of symptoms is particularly concerning for a life-threatening emergency, such as a ruptured aortic aneurysm, as it can lead to shock.

While other conditions such as diverticulitis, renal colic, and lumbar disk herniation can cause back pain, they typically do not present with hypotension. Diverticulitis is more commonly associated with gastrointestinal symptoms, renal colic typically presents with flank pain radiating to the groin without hypotension, and lumbar disk herniation usually presents with radiculopathy or neurologic symptoms rather than hypotension. Therefore, the clinical picture aligns closely with a ruptured aortic aneurysm being the most likely diagnosis.