**Core Concept**
The clinical presentation of a 2-month-old baby with jaundice, turmeric-colored urine, and pale stools, along with an enlarged liver span, is suggestive of biliary atresia. Biliary atresia is a congenital or acquired obstruction of the bile ducts, leading to bile accumulation in the liver, causing jaundice, and liver damage.

**Why the Correct Answer is Right**
The definitive diagnosis of biliary atresia is established by demonstrating the absence of bile ducts or their obstruction. **Hepatobiliary iminodiacetic acid (HIDA) scan**, also known as a cholescintigraphy, is the most specific investigation for establishing the diagnosis. This imaging test involves the intravenous administration of a radioactive tracer that is taken up by the liver and excreted into the bile. In cases of biliary atresia, the tracer does not flow into the intestines, indicating a blockage in the bile ducts.

**Why Each Wrong Option is Incorrect**
**Option A:** Ultrasound of the abdomen may show an enlarged liver and bile duct dilatation, but it cannot definitively diagnose biliary atresia.
**Option B:** Liver function tests (LFTs) may indicate liver damage, but they are non-specific and cannot confirm the diagnosis of biliary atresia.
**Option C:** Abdominal X-ray may show an enlarged liver, but it is not specific for diagnosing biliary atresia.

**Clinical Pearl / High-Yield Fact**
Biliary atresia is a medical emergency that requires prompt diagnosis and treatment to prevent liver cirrhosis and death.

**Correct Answer:** C. HIDA scan