Phototherapy lowers serum bilirubin levels in neonates. What is the underlying mechanism?

• A It induces hepatic UDP glucuronyl transferase activity, accelerating conjugation
• B It enhances albumin binding capacity, shifting bilirubin out of tissues
• C It increases red cell lifespan by reducing haemolysis
• D It converts bilirubin into water soluble configurational and structural isomers that are excreted without conjugation ✓

Explanation

Light of wavelength around 450 to 465 nm converts toxic native Z,Z bilirubin into E,Z configurational isomers and lumirubin, a stable structural isomer. These isomers are water soluble and are excreted directly in bile and urine without needing glucuronidation. Phototherapy does not induce glucuronyl transferase (that requires phenobarbital), does not alter red cell survival, and its benefit comes from excretion of altered pigment rather than from changes in albumin binding.

Reference: Nelson Textbook of Pediatrics, 21st ed.

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