**Question:** Pigment providing colour to stool is -

A. Bilirubin
B. Hemoglobin
C. Mucus
D. Pigments from food

**Core Concept:** Stool color is primarily determined by the presence of bilirubin, a breakdown product of heme catabolism. Bilirubin and other pigments are transported in the blood by binding to proteins.

**Why the Correct Answer is Right:** Bilirubin is the primary pigment responsible for the color of stool because it is the product of heme catabolism, which occurs in the liver. Bilirubin is conjugated (glucuronidated) by the liver and then secreted into the bile ducts. In the small intestine, bile is secreted into the duodenum, where it mixes with food and water, and the conjugated bilirubin gets released into the large intestine (colon). Due to the acidic environment, the glucuronide groups are cleaved, and bilirubin gets reabsorbed into the circulation via mucosal cells. The unbound (free) bilirubin is then excreted in the stool.

**Why Each Wrong Option is Incorrect:**

A. **Bilirubin (A):** Bilirubin is conjugated and gets excreted into the bile and then the large intestine, where it is reabsorbed. Therefore, it is not present in stool.

B. **Hemoglobin (B):** Hemoglobin is a protein found in red blood cells and does not contribute to stool color as it is not present in the large intestine. Hemoglobin is broken down in the spleen and released into circulation, where it is cleared by the liver and excreted in urine.

C. **Mucus (C):** Mucus is a gel-like substance secreted by intestinal mucus cells. While it is present in the large intestine, it does not contribute to the color of stool as it is a different color (white/translucent) and is not a pigment.

D. **Pigments from food (D):** While food can influence the appearance of the stool (for example, darker colored foods like beets or spinach), the primary determinant of stool color is bilirubin. Food pigments are not responsible for the color of stool.

**Clinical Pearl:** Bilirubin is a yellow pigment, so a yellow-colored stool indicates a problem in bilirubin clearance and should raise suspicion of liver disease or jaundice. Stools can also be influenced by dietary factors, but the primary determinant of stool color is bilirubin.