**Core Concept**
Neonatal cholestasis refers to the impaired bile flow in newborns, which can be caused by various factors including genetic disorders, infections, and anatomical abnormalities. Elevated serum gamma-glutamyltransferase (GGT) is a marker of cholestasis, indicating bile duct obstruction or liver cell injury.

**Why the Correct Answer is Right**
Gamma-glutamyltransferase (GGT) is an enzyme present in the bile ducts and hepatocytes. Elevated GGT levels (>600 IU/L) in neonatal cholestasis suggest bile duct obstruction or liver cell injury. This is because GGT is released from damaged bile ducts or hepatocytes into the bloodstream. In conditions like biliary atresia, a congenital obstruction of the bile ducts, GGT levels are significantly elevated due to the release of this enzyme from damaged bile ducts.

**Why Each Wrong Option is Incorrect**
**Option A:** This option is not provided, so we'll skip it.
**Option B:** While other conditions like neonatal hepatitis may also cause elevated GGT, biliary atresia is the most likely diagnosis with GGT > 600 IU/L.
**Option C:** Alagille syndrome, a genetic disorder affecting the bile ducts, may cause cholestasis but is not the most likely diagnosis with GGT > 600 IU/L.
**Option D:** This option is not provided, so we'll skip it.

**Clinical Pearl / High-Yield Fact**
In neonatal cholestasis, elevated GGT levels (>600 IU/L) are highly suggestive of biliary atresia, a condition requiring prompt surgical intervention to prevent liver cirrhosis. Remember the mnemonic "Biliary Atresia: Babies Are Seriously Ill" to recall this high-yield fact.

**Correct Answer: B. Biliary atresia.**