As a reduction product of bilirubin catabolism, which compound is partially reabsorbed from the intestine via the portal circulation for reexcretion by the liver?

• A. Verdohemoglobin
• B. Urobilinogen
• C. Urobilin
• D. Biliverdin

Answer: (B)

The main idea is how bilirubin metabolites recycle through the enterohepatic circulation. After bilirubin is conjugated in the liver and sent to the intestine, gut bacteria reduce it to a compound called urobilinogen. A portion of this reduced product is small enough to be reabsorbed from the intestine into the portal blood and carried back to the liver, where it can be re-secreted into bile. This recycling helps maintain bilirubin turnover and explains why certain bilirubin-related pigments appear in both stool and urine.

Urobilinogen is the reduction product formed in the gut, and its partial reabsorption via the portal circulation for reexcretion by the liver is characteristic of this recycling process. Some of the reabsorbed urobilinogen can be converted further and appear in urine as urobilin, while most of the remaining urobilinogen in the gut is converted to stercobilin, giving feces its color.

Verdohemoglobin and biliverdin are upstream intermediates in heme breakdown, not the products primarily involved in this intestinal reabsorption and hepatic reexcretion cycle. Urobilin is produced from urobilinogen and is typically excreted in urine, but the step being asked about—reabsorption after intestinal reduction for return to the liver—points to urobilinogen.