**Core Concept**
Hepatic encephalopathy is a neuropsychiatric disorder that arises from liver dysfunction, leading to the accumulation of toxic substances in the brain. This condition is characterized by a spectrum of cognitive and motor abnormalities, ranging from mild confusion to coma.

**Why the Correct Answer is Right**
Hepatic encephalopathy is caused by the accumulation of ammonia and other toxins in the blood, which are normally cleared by the liver. These toxins can cross the blood-brain barrier and cause damage to brain cells, leading to the characteristic symptoms of hepatic encephalopathy. The liver's inability to detoxify these substances is a key factor in the development of this condition.

**Why Each Wrong Option is Incorrect**
**Option A:** Ammonia levels are not a direct measure of the severity of hepatic encephalopathy. While ammonia is a key contributor to the condition, other factors such as the presence of other toxins and the individual's underlying liver function also play a role.

**Option B:** Hepatic encephalopathy can occur in patients with any degree of liver dysfunction, not just those with advanced cirrhosis. Early stages of liver disease can also lead to the development of hepatic encephalopathy.

**Option C:** Lactulose is a common treatment for hepatic encephalopathy, but it is not the only option. Other treatments, such as rifaximin and lactitol, may also be used to manage this condition.

**Clinical Pearl / High-Yield Fact**
A key aspect of managing hepatic encephalopathy is to identify and treat the underlying cause of liver dysfunction. This may involve addressing issues such as viral hepatitis, alcohol abuse, or non-alcoholic fatty liver disease.

**Correct Answer: D**