Which type of bilirubin is known as unconjugated?

Indirect bilirubin is known as unconjugated bilirubin because it is the form of bilirubin that has not yet undergone the biochemical process of conjugation in the liver. This form is lipid-soluble and can diffuse across cell membranes, including the blood-brain barrier, which can make it potentially toxic if levels are elevated.

In the body, unconjugated bilirubin is produced from the breakdown of heme in red blood cells. It is transported to the liver, where it is converted into conjugated bilirubin, or direct bilirubin, by undergoing a process called glucuronidation. This conversion allows bilirubin to be water-soluble and more easily excreted from the body through bile and urine.

The other options represent forms of bilirubin that have undergone processing or modification. Direct bilirubin refers to the conjugated form that is water-soluble, while bilirubin glucuronide specifically denotes its glucuronide conjugate, and conjugated bilirubin is essentially synonymous with direct bilirubin. Thus, indirect bilirubin clearly stands out as the only form referred to as unconjugated.