Final answer:
Ascites in cirrhosis, often related to alcohol abuse, is most likely due to impaired portal venous return caused by cirrhosis-induced portal hypertension.
Step-by-step explanation:
The most likely cause of ascites in a client with a long history of alcohol abuse who developed cirrhosis of the liver is impaired portal venous return. Cirrhosis leads to the formation of fibrous tissue in the liver, replacing dead liver cells, which can be caused by excessive alcohol consumption, among other factors. This scarring obstructs the passage of blood through the liver, leading to portal hypertension. Portal hypertension, in turn, is a primary factor for the development of ascites, as it hinders blood from the intestines from properly traveling through the liver and back to the heart.