Final answer:
In a situation of complete biliary obstruction, there is a blockage that prevents bile, carrying the water-soluble bilirubin diglucuronide, from reaching the intestine. Therefore, no urobilinogen will be formed in the intestine from bilirubin, and ultimately, there will be an absence of urobilinogen in both urine and stool. Thus, the correct answer to the question is b) Urobilinogen.
Step-by-step explanation:
The student's question relates to what substance is absent from urine and stool in the case of complete biliary obstruction. With complete biliary obstruction, the hepatic or common bile ducts are blocked, preventing conjugated bile pigments like bilirubin glucuronide from being excreted. As a result, these pigments can back up into the bloodstream and may appear in the urine, causing choluric jaundice.
During normal liver function, bilirubin is transported to the liver, where it is conjugated to form bilirubin glucuronide, making it water-soluble and excretable. In the intestine, most of this converted bilirubin is changed into stercobilinogen and then into stercobilin, which imparts the brown color to feces. Some of it is also converted into urobilinogen in the intestine; a portion of this is reabsorbed and later excreted in the urine as urobilin, giving urine its yellow color.
In the case of complete biliary obstruction, urobilinogen is absent from urine and stool because the blockage prevents bilirubin glucuronide from reaching the intestine to be converted into urobilinogen.