Which type of bilirubin is bound to albumin and cannot be discarded?

Unconjugated bilirubin is the form of bilirubin that is bound to albumin and cannot be discarded easily by the body. This type of bilirubin is produced from the breakdown of hemoglobin in red blood cells. It is lipid-soluble and hence must bind to albumin for transport in the bloodstream to the liver, where it can undergo conjugation. The albumin binding is vital because, without this binding, the unconjugated bilirubin would be too insoluble to be transported in blood and would lead to toxicity.

The liver then conjugates the unconjugated bilirubin, converting it into a more water-soluble form, which can be excreted in bile (known as conjugated bilirubin). In contrast, conjugated bilirubin is water-soluble and can be more readily excreted from the body. Other terms mentioned, such as direct bilirubin, refer to the conjugated form and inorganic bilirubin does not apply to the standard categories of bilirubin in clinical practice. Thus, the key characteristic of unconjugated bilirubin being highly reliant on albumin binding is what makes it the correct answer to this question.