Final answer:
The patient's high risk for chronic liver disease, likely cirrhosis, and presentation of jaundice, a distended abdomen, hematemesis, and hemodynamic instability suggest variceal hemorrhage as the cause of the acute upper gastrointestinal bleeding (UGIB).
Step-by-step explanation:
In this case, the patient is a high risk for chronic liver disease, likely cirrhosis, due to alcoholism and intravenous drug abuse (IVDA). The patient presents with jaundice, a distended abdomen, and hematemesis, which are suggestive of acute significant upper gastrointestinal bleeding (UGIB). Given the patient's social history and abdominal exam finding, variceal hemorrhage is more likely as the cause of the acute UGIB.