**Core Concept**
Hepatic coma, also known as hepatic encephalopathy, is a neurological complication of liver disease characterized by altered mental status, ranging from mild confusion to coma. It occurs due to the accumulation of toxins, primarily ammonia, in the blood, which affects brain function.

**Why the Correct Answer is Right**
The correct blood test to confirm a diagnosis of hepatic coma is the **arteriovenous difference (AVD) in ammonia levels**. In hepatic coma, the liver's ability to detoxify ammonia is impaired, leading to elevated levels of ammonia in the blood. The AVD in ammonia levels measures the difference between arterial and venous ammonia levels. In patients with hepatic coma, the AVD is typically increased, indicating impaired hepatic clearance of ammonia.

**Why Each Wrong Option is Incorrect**
* **Option A:** Serum bilirubin levels may be elevated in liver disease, but they do not directly confirm hepatic coma.
* **Option B:** Prothrombin time (PT) may be prolonged in liver disease due to impaired synthesis of clotting factors, but it is not a specific marker for hepatic coma.
* **Option C:** Liver function tests (LFTs) such as aspartate aminotransferase (AST) and alanine aminotransferase (ALT) may be elevated in liver disease, but they do not directly confirm hepatic coma.

**Clinical Pearl / High-Yield Fact**
Hepatic encephalopathy can be precipitated by factors such as constipation, electrolyte imbalances, and infections, which can further increase ammonia levels. Early recognition and management of these precipitating factors are crucial in preventing the progression of hepatic encephalopathy.

**Correct Answer:** C.