What type of bilirubin is unconjugated and lipid soluble?

Unconjugated bilirubin, also known as indirect bilirubin, is formed from the breakdown of heme in red blood cells. This form of bilirubin is not water-soluble; rather, it is lipid-soluble and unable to be excreted directly into the bile. Due to its lipid solubility, unconjugated bilirubin can easily cross cell membranes, including the blood-brain barrier, which is why elevated levels can lead to kernicterus in neonates.

In contrast, conjugated bilirubin is water-soluble and forms when unconjugated bilirubin is processed in the liver, where it binds to glucuronic acid, making it easier for the body to excrete. Direct bilirubin refers to this conjugated form that is processed and can be measured in blood tests. Free bilirubin is often synonymous with unconjugated bilirubin but is not the most commonly used term in clinical settings.

Therefore, the characteristic features of indirect bilirubin—its unconjugated status and lipid solubility—make it the correct choice.