Which of the following is the primary mechanism of unconjugated hyperbilirubinemia in physiological jaundice of the newborn?

Question

Accepted Answer

A. Immature hepatic glucuronyl transferase activity with increased enterohepatic circulation. ## Mechanism of Physiological Jaundice

**Key Point:** Physiological jaundice results from a combination of immature hepatic conjugation capacity and increased enterohepatic circulation of bilirubin in the newborn.

### Pathophysiology

1. **Immature UDP-glucuronosyltransferase (UGT1A1)** — The neonatal liver has only 1% of adult enzyme activity at birth, reaching adult levels by 2–4 weeks of age.
2. **Increased enterohepatic circulation** — Neonates have:
   - High intestinal β-glucuronidase activity
   - Sterile gut (lack of bacteria to convert bilirubin to urobilinogen)
   - Increased reabsorption of unconjugated bilirubin from the intestine

### Timeline & Clinical Features

- Appears after 24 hours of life (rarely in first 24 hours)
- Peaks at 3–5 days of life
- Resolves by 1–2 weeks in term infants
- Does NOT cause kernicterus if managed appropriately

**High-Yield:** Physiological jaundice is **unconjugated** hyperbilirubinemia; conjugated bilirubin remains normal.

### Why Other Options Are Wrong

| Mechanism | Why Not Physiological Jaundice |
|-----------|--------------------------------|
| Hemolysis | Occurs in pathological jaundice (ABO/Rh incompatibility, G6PD deficiency); RBC lifespan is normal in physiological jaundice |
| Biliary atresia | Causes **conjugated** (direct) hyperbilirubinemia, not unconjugated; presents later with acholic stools |
| Hepatocellular injury | Seen in TORCH infections, syphilis; causes mixed hyperbilirubinemia with elevated transaminases |

**Clinical Pearl:** Early feeding (within 2 hours of birth) and adequate breastfeeding reduce enterohepatic circulation and lower peak bilirubin levels by 10–15%.

![Neonatal Jaundice diagram](https://mmcphlazjonnzmdysowq.supabase.co/storage/v1/object/public/blog-images/explanation/3492.webp)

Answer

B. Biliary atresia with complete obstruction of bile flow

Answer

C. Increased hemolysis due to shortened RBC lifespan

Answer

D. Direct hepatocellular injury from intrauterine infection

Which of the following is the primary mechanism of unconjugated hyperbilirubinemia in physiological jaundice of the newborn?

Explanation

Mechanism of Physiological Jaundice

Pathophysiology

Timeline & Clinical Features

Why Other Options Are Wrong

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Mechanism	Why Not Physiological Jaundice
Hemolysis	Occurs in pathological jaundice (ABO/Rh incompatibility, G6PD deficiency); RBC lifespan is normal in physiological jaundice
Biliary atresia	Causes conjugated (direct) hyperbilirubinemia, not unconjugated; presents later with acholic stools
Hepatocellular injury	Seen in TORCH infections, syphilis; causes mixed hyperbilirubinemia with elevated transaminases