What type of bilirubin is considered fat-soluble and toxic to tissues?

Unconjugated bilirubin is the correct answer because it is the form of bilirubin that is fat-soluble and can potentially be toxic to tissues. Unconjugated bilirubin is created from the breakdown of heme in red blood cells and is not water-soluble, which allows it to be absorbed by fatty tissues. Elevated levels of unconjugated bilirubin can lead to conditions such as kernicterus, a form of brain damage that is a result of excess bilirubin crossing the blood-brain barrier.

In contrast, conjugated bilirubin is the result of the liver processing unconjugated bilirubin, making it water-soluble, and suitable for excretion through bile. This conversion helps to eliminate bilirubin from the body and reduces its potential toxicity. Urobilin is a byproduct of the breakdown of conjugated bilirubin, which further emphasizes that it is not the form that is fat-soluble or toxic. Understanding the differences between these forms of bilirubin is crucial for managing conditions like jaundice in neonates.