A 54-year-old patient, a heavy drinker, who 4 years ago had a case of viral hepatitis B, has been experiencing heartburn and a burning pain retrosternal for the past two months. In the morning, after eating and lifting a heavy object, he had an episode of vomiting with fresh dark blood. His skin is pale and moist, pulse – 92/min., blood pressure – 90/60 mm Hg. His sclerae are icteric. His abdomen is enlarged due to ascites and hepatosplenomegaly. What is the most likely cause of bleeding in this case?