Which bilirubin form is fat-soluble before conjugation?

• A. Conjugated bilirubin is fat-soluble.
• B. Unconjugated bilirubin is fat-soluble.
• C. Both forms are fat-soluble.
• D. Neither form is fat-soluble.

Answer: (B)

Before conjugation, bilirubin exists as unconjugated bilirubin, which is fat-soluble. Its lipophilic nature means it travels in the bloodstream largely bound to albumin and must be carried to the liver for processing. In the liver, it’s conjugated with glucuronic acid to form conjugated bilirubin, which is water-soluble and excreted into bile. So the fat-soluble form prior to conjugation is unconjugated bilirubin. The conjugated form is already water-soluble, so it isn’t fat-soluble.