**Core Concept**
Hepatic encephalopathy is a neuropsychiatric disorder resulting from liver dysfunction, leading to the accumulation of toxic substances in the blood. The liver plays a crucial role in detoxification, and its failure can cause an increase in circulating toxins.

**Why the Correct Answer is Right**
The correct answer is ammonia (NH3). Ammonia is a potent neurotoxin that accumulates in the blood when the liver is unable to detoxify it. The liver normally converts ammonia into urea, which is then excreted in the urine. However, in liver failure, this process is impaired, leading to elevated ammonia levels. Ammonia crosses the blood-brain barrier and binds to glutamate receptors, causing neuroexcitotoxicity and leading to the clinical manifestations of hepatic encephalopathy.

**Why Each Wrong Option is Incorrect**
* **Option A:** This option is not specified, as the question was incomplete. However, if we assume it's another toxin, it would still not be the primary cause of hepatic encephalopathy, as ammonia is the key player.
* **Option B:** Other toxins, such as mercaptans and phenols, may also accumulate in liver failure, but they are not the primary cause of hepatic encephalopathy.
* **Option C:** This option is not specified, but even if it were another substance, it would not be the primary cause of hepatic encephalopathy, as ammonia is the primary neurotoxin involved.

**Clinical Pearl / High-Yield Fact**
The severity of hepatic encephalopathy can be graded using the West Haven criteria, which include symptoms such as minimal hepatic encephalopathy (MHE), overt hepatic encephalopathy (OHE), and hepatic coma. The treatment of hepatic encephalopathy focuses on reducing ammonia production, promoting its clearance, and managing precipitating factors.

**Correct Answer: C. Ammonia.