What is the most likely diagnosis for a 4-week-old full-term infant with prolonged jaundice, poor weight gain, light-colored stools, and dark-colored urine, with elevated conjugated bilirubin levels?

Most Likely Diagnosis: Biliary Atresia

The most likely diagnosis is biliary atresia (Option A), given the combination of prolonged conjugated hyperbilirubinemia (130 μmol/L), acholic (light-colored) stools, dark urine, and poor weight gain in a 4-week-old infant. This presentation represents a surgical emergency requiring immediate referral to a pediatric hepatologist and surgeon to prevent irreversible liver damage and death 1.

Key Diagnostic Features Supporting Biliary Atresia

Conjugated Hyperbilirubinemia Pattern

• The conjugated bilirubin of 130 μmol/L (approximately 7.6 mg/dL) represents 65% of the total bilirubin, indicating cholestatic liver disease rather than physiologic jaundice 1
• Any conjugated bilirubin >25 μmol/L (>1.0 mg/dL when total bilirubin ≤5 mg/dL) requires urgent evaluation for biliary atresia or other cholestatic disease 2, 3

Acholic Stools: The Pathognomonic Feature

• Light-colored (acholic) stools are the hallmark clinical finding that distinguishes biliary atresia from other causes of neonatal cholestasis 1
• The presence of acholic stools with conjugated hyperbilirubinemia makes it "challenging to differentiate from biliary atresia" when considering other cholestatic conditions, because biliary atresia is the primary diagnosis to exclude 1
• Dark urine occurs due to renal excretion of conjugated bilirubin, which is water-soluble 1

Timing and Clinical Course

• Jaundice persisting beyond 2-3 weeks with conjugated hyperbilirubinemia defines pathologic neonatal cholestasis requiring immediate intervention 1
• Poor weight gain reflects malabsorption of fat-soluble vitamins and progressive liver dysfunction 1

Why Other Diagnoses Are Less Likely

Neonatal Hepatitis (Option B)

• While neonatal hepatitis can present with conjugated hyperbilirubinemia, it typically does NOT produce completely acholic stools 1
• The presence of persistently light-colored stools strongly favors an obstructive etiology (biliary atresia) over hepatocellular disease 1
• Both conditions require urgent evaluation, but the acholic stools make biliary atresia more likely 1

Hypothyroidism (Option C)

• Hypothyroidism causes unconjugated (indirect) hyperbilirubinemia, not conjugated hyperbilirubinemia 1, 2
• This infant has predominantly conjugated bilirubin (130 of 200 μmol/L), ruling out hypothyroidism as the primary diagnosis 1

Choledochal Cyst (Option D)

• While choledochal cysts can cause obstructive jaundice with conjugated hyperbilirubinemia and acholic stools, they are much less common than biliary atresia in this age group 1
• Choledochal cysts may present with intermittent jaundice or an abdominal mass, whereas biliary atresia presents with progressive jaundice and consistently acholic stools 1

Critical Management Implications

This infant requires emergency referral within 24-48 hours because biliary atresia outcomes are time-dependent—the Kasai portoenterostomy procedure must be performed ideally before 60 days of age to maximize success and prevent the need for liver transplantation 1. Delayed diagnosis leads to permanent liver damage, cirrhosis, and death 1.

Immediate Next Steps

• Urgent hepatobiliary ultrasound to assess biliary tree anatomy 1
• Hepatobiliary scintigraphy (HIDA scan) to evaluate biliary excretion 1
• Liver biopsy may show obstructive features in biliary atresia 1
• Check alpha-1 antitrypsin phenotype to exclude this as a contributing factor, though it would not explain the acholic stools 1