Final answer:
Lactulose is effective in treating cirrhosis when it improves mental status due to reduced ammonia levels. The primary indicator of its efficacy is the patient being alert and oriented, indicative of lower ammonia levels that can contribute to hepatic encephalopathy.
Step-by-step explanation:
The question pertains to understanding the effectiveness of lactulose in treating an older adult with advanced cirrhosis. Lactulose is a synthetic sugar used to treat constipation and to reduce the high levels of ammonia seen in patients with cirrhosis, which can lead to hepatic encephalopathy—a decline in brain function due to severe liver disease. When lactulose works effectively, it lowers ammonia levels by converting it into ammonium in the gut, which is then excreted through the stool. Hence, the primary indicator of lactulose's effectiveness in a patient with advanced cirrhosis is improved mental status, which would present as the patient being alert and oriented. This corresponds to option A. Bilirubin levels (option C) would be more directly affected by the overall liver function, while nausea or anorexia (option B) and frequency of stools (option D) are less specific indicators of lactulose efficacy for ammonia clearance.