Question

Yellow colour of the urine is due to Urochrome, which is synthesized by the oxidation of
(a) Stercobilin
(b) Uric Acid
(c) Bile
(d) Cholesterol

Answer 1

Hint: Urochrome is the chemical responsible for the yellow colouration of urine. It has a linear tetrapyrrole structure, along with related compound urobilinogen. They are the final degradation products of the cyclic tetrapyrrole rings of haem. They are the oxidation products of the fluid stored in the gallbladder.

Complete answer:
Haem is a tetrapyrrole compound that forms the skeleton for hemoglobin. After the life cycle of an RBC is completed, the haem of the RBCs is metabolized. The haem is first degraded into bilirubin and biliverdin and then excreted as bile. Bile is further degraded to form urobilinogen by the microbes present in the large intestine. Some of the urobilinogen remains in the large intestine, and it is converted to stercobilin that gives feces its brown colouration. Some of it is reabsorbed back into the bloodstream which is then delivered to the kidney. When urobilinogen gets exposed to air, it is oxidized to urobilin or urochrome, which gives urine its yellow colour.
Many urine analysis tests monitor the amount of urochrome in urine, as its levels give an insight into the effectiveness of the urinary tract function. In normal conditions, urine would appear as either light yellow or colourless. In cases of lack of water intake, for example, following sleep or dehydration, the water content of urine is reduced, which leads to increased concentration of urochrome, producing a darker colour of the urine.
So, the correct answer is, ‘Bile.’

Note: Conditions like obstructive jaundice reduce biliary excretion, which is then excreted directly from the bloodstream instead of the urine. This gives dark-coloured urine but with a low urochrome concentration, no urobilinogen, and also usually with correspondingly pale feces. Darker urine can also be seen because of other chemicals, such as various drugs, porphyrins in patients with porphyria, etc.