Which type of bilirubin is bound to protein (albumin)?

• A. Conjugated bilirubin
• B. Total bilirubin
• C. Unconjugated bilirubin
• D. Delta bilirubin

Answer: (C)

Unconjugated bilirubin is the form of bilirubin that is bound to proteins, particularly albumin. This form is produced from the breakdown of hemoglobin when red blood cells are eliminated from circulation. Unconjugated bilirubin is lipid-soluble and therefore cannot be excreted directly from the body. It travels through the bloodstream bound to albumin to the liver, where it undergoes conjugation.

In contrast, conjugated bilirubin, which is water-soluble, does not bind to proteins in the same way; instead, it is excreted into bile and then into the intestines. Total bilirubin is a measure that includes both conjugated and unconjugated forms but does not distinguish between them. Delta bilirubin is a form of conjugated bilirubin that is bound to albumin, but it is not the primary protein-bound form addressed in this question. Thus, the correct answer focuses specifically on the unconjugated form and its relationship with protein binding.