What is conjugated bilirubin, and how is it formed and excreted?

• A. Bilirubin conjugated with glucuronic acid in the liver by UDP-glucuronyl transferase; excreted in bile and stool.
• B. Bilirubin conjugated in the intestine by lactase; excreted in urine.
• C. Bilirubin conjugated with sulfate in the kidney; excreted in urine.
• D. Bilirubin remains unconjugated and is excreted in bile.

Answer: (A)

Conjugated bilirubin is the water-soluble form that the body eliminates. In the liver, bilirubin, which comes from the breakdown of heme, is taken up by hepatocytes and conjugated with glucuronic acid by the enzyme UDP-glucuronyl transferase. This creates bilirubin diglucuronide, a soluble molecule known as direct (conjugated) bilirubin. Because it is water-soluble, it can be secreted into bile and then travel into the intestinal tract and be excreted in the stool. Some of the conjugated bilirubin reaching the intestine is acted on by gut bacteria, forming urobilinogen, which can be reabsorbed and later excreted in urine, but the primary excretion route for conjugated bilirubin is bile into the stool. The other options are not correct because conjugation does not occur in the intestine by lactase, nor in the kidney by sulfate attachment, and bilirubin is not excreted in bile in its unconjugated form.