**Core Concept**
Neonatal cholestasis refers to a condition where there is a reduction or stoppage of bile flow in newborns, often due to anatomical or functional obstruction. The serum gamma-glutamyl transpeptidase (GGT) is an enzyme found in liver cells and is elevated in conditions where there is cholestasis, particularly those involving bile duct obstruction.

**Why the Correct Answer is Right**
An elevated serum GGT level greater than 600 IU/L in neonatal cholestasis is highly suggestive of biliary atresia, a congenital condition characterized by the absence or obliteration of bile ducts. This leads to bile accumulation in the liver, causing damage and scarring. The elevated GGT level is a result of the liver's attempt to compensate for the bile duct obstruction by releasing more of this enzyme. In biliary atresia, the serum GGT level is often significantly elevated, making it a key diagnostic marker.

**Why Each Wrong Option is Incorrect**
**Option A:** This option is incorrect because while other causes of neonatal cholestasis, such as parenteral nutrition-associated cholestasis or idiopathic cholestasis, may have elevated GGT levels, they typically do not reach levels as high as 600 IU/L.

**Option B:** This option is incorrect because congenital hypoplasia of the pancreas or biliary tree may be associated with elevated GGT levels, but the levels are usually not as high as those seen in biliary atresia.

**Option C:** This option is incorrect because neonatal hepatitis caused by infections such as CMV or HSV may have elevated GGT levels, but the levels are usually not as high as those seen in biliary atresia.

**Option D:** This option is incorrect because Alagille syndrome is a genetic disorder affecting the bile ducts, but it typically presents with lower GGT levels compared to biliary atresia.

**Clinical Pearl / High-Yield Fact**
A key clinical finding in biliary atresia is the presence of pale, firm liver with a "nutmeg" appearance due to bile duct proliferation and fibrosis. This finding can be seen on abdominal ultrasound and is a critical diagnostic clue.

**Correct Answer:** C. Biliary atresia.