Which mechanism best explains how phototherapy lowers serum bilirubin levels in a neonate?
- A Photoisomerisation of bilirubin into water soluble geometric isomers such as lumirubin ✓
- B Induction of hepatic UDP-glucuronosyltransferase enzyme activity
- C Enhanced enterohepatic circulation of conjugated bilirubin
- D Direct photodestruction of bilirubin into carbon dioxide and water
Correct answer: A. Photoisomerisation of bilirubin into water soluble geometric isomers such as lumirubin
Explanation
Light absorbed by skin bilirubin causes configurational isomerisation to the 4Z,15E isomer and structural isomerisation to lumirubin. Both are water soluble and can be excreted in bile and urine without prior glucuronidation, bypassing the immature hepatic conjugating system of the neonate. Enzyme induction by light does not occur, that effect belongs to phenobarbital. Enterohepatic circulation increases bilirubin reabsorption rather than lowering it, and bilirubin is not degraded into carbon dioxide and water.
Reference: Nelson Textbook of Pediatrics, 21st ed.
High-yield for: NEET PGINI-CETNExTFMGEUSMLEPLABMRCP
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