**Core Concept**
Acute liver failure (ALF) in children is a life-threatening condition characterized by rapid deterioration of liver function, leading to coagulopathy, encephalopathy, and multi-organ failure. The prognosis of ALF is influenced by various serum biochemical tests, including liver enzymes, bilirubin, and coagulation parameters.

**Why the Correct Answer is Right**
The most important abnormal serum biochemical test indicating poor prognosis in ALF is the **International Normalized Ratio (INR)**, which reflects the level of coagulopathy. Elevated INR values (> 3.0) are associated with increased risk of bleeding, cerebral edema, and mortality. This is because INR measures the extrinsic and common coagulation pathways, which are affected in ALF due to decreased production of coagulation factors by the liver. Moreover, INR is a more sensitive indicator of liver dysfunction than liver enzymes or bilirubin levels in the early stages of ALF.

**Why Each Wrong Option is Incorrect**
* **Option A:** Elevated serum ammonia levels are a marker of hepatic encephalopathy, but they do not directly indicate poor prognosis in ALF.
* **Option B:** Elevated serum lactate levels may indicate metabolic acidosis, but they are not a specific indicator of poor prognosis in ALF.
* **Option C:** Elevated serum bilirubin levels are a marker of cholestasis, but they do not directly indicate poor prognosis in ALF.

**Clinical Pearl / High-Yield Fact**
In children with ALF, a high INR (> 3.0) is a critical prognostic indicator, and prompt reversal with fresh frozen plasma or other coagulation factor concentrates is essential to prevent bleeding complications.

**Correct Answer:** C. Elevated INR.