Human faeces normally appears yellowish brown in colour; this colour is mainly due to the presence of which bile pigment derivative formed from the breakdown of haemoglobin?

Introduction / Context:
The characteristic yellowish brown colour of human faeces is not random. It reflects the presence of pigments formed during the breakdown and excretion of haemoglobin from old red blood cells. This question asks you to identify the pigment responsible for this colour. Understanding the role of bile pigments and their derivatives in waste elimination links digestive physiology with blood cell turnover.

Given Data / Assumptions:
- Old red blood cells are broken down in the liver and spleen, releasing haemoglobin.- Haemoglobin is converted to bile pigments that enter the intestine with bile.- Intestinal bacteria modify these pigments into compounds that colour faeces and urine.- The options list several enzymes and one pigment, urobilin.

Concept / Approach:
When red blood cells are broken down, haem is converted to biliverdin and then bilirubin in the liver. Bilirubin is secreted into bile and released into the intestine. There, microbial action converts bilirubin into urobilinogen and related products. Some of these are oxidised to urobilin and stercobilin, which give urine and faeces their characteristic colours. Urobilin and stercobilin are pigments, not enzymes. Pepsin, renin and amylase are digestive enzymes with no colouring role. Therefore, among the given options, urobilin is the pigment associated with the yellowish colour of faeces.

Step-by-Step Solution:
Step 1: Recall that bile pigments originate from the breakdown of haemoglobin from aged red blood cells.Step 2: Understand that these pigments are modified by intestinal bacteria into urobilinogen and related compounds.Step 3: Recognise that oxidation of these compounds yields pigments such as urobilin, which colour excretory products.Step 4: Examine the options and identify which one is a pigment rather than a digestive enzyme or structural protein.Step 5: Select option D, urobilin, a pigment derived from bile pigments, as the correct answer.

Verification / Alternative check:
Physiology references describe how haemoglobin breakdown produces bilirubin, which is transported in bile to the intestine.They also explain that intestinal conversion of bilirubin leads to pigments such as urobilin and stercobilin, which colour faeces and urine.No text attributes faecal colour to enzymes like pepsin or amylase, confirming that urobilin is the relevant pigment among the choices.

Why Other Options Are Wrong:
Option A is wrong because pepsin is a gastric enzyme that digests proteins and is not a pigment.Option B is wrong because renin is an enzyme involved in milk coagulation in some young animals and does not determine faecal colour.Option C is wrong because amylase digests starch and has no pigment function.Option E is wrong because keratin is a structural protein in hair, nails and skin, not a pigment in digestive waste.

Common Pitfalls:
Students may misunderstand the role of enzymes versus pigments and assume that any common digestive enzyme might influence colour.Another pitfall is to forget that breakdown of haemoglobin produces bile pigments that ultimately colour both faeces and urine.Linking the word urobilin with urine and faecal pigments can help you remember that it is a bile related pigment, not a digestive enzyme.

Final Answer:
The yellowish brown colour of human faeces is mainly due to the presence of urobilin, a pigment derived from bile pigments formed during haemoglobin breakdown.