Final answer:
The colorless end product of bilirubin metabolism is stercobilinogen, which is later converted to stercobilin, responsible for the brown color of feces. The correct answer to the question is a) Urobilinogen.
Step-by-step explanation:
Metabolism of Bilirubin and its Colorless End Product
The metabolism of bilirubin involves its conversion from a water-insoluble form to a water-soluble form in the liver. Initially, bilirubin is transported to the liver complexed with albumin. Once in the liver, it is conjugated with glucuronic acid to form bilirubin diglucuronide by the action of UDP glucuronyl transferase. This water-soluble form of bilirubin is then excreted into the intestine. In the intestine, the majority of bilirubin is metabolized by bacteria into stercobilinogen, a colourless compound that is further converted into stercobilin, providing a brown colour to faeces. A small proportion of the metabolized bilirubin is reabsorbed into the bloodstream and excreted from the body as urobilin, a pigment that imparts the yellow colour to urine. In summary, the colourless end product of bilirubin metabolism is stercobilinogen, which is then converted to stercobilin, imparting colour to faeces. While urobilinogen, another metabolite, is eventually excreted in the urine, it is not colourless. Therefore, the correct answer to the student's question, "What is a colourless end product of bilirubin metabolism?" is a) Urobilinogen.