## Core Concept
Bilirubin is a yellow compound that occurs in the normal catabolic pathway that breaks down heme in red blood cells. The majority of bilirubin is produced from the breakdown of **old red blood cells (RBCs)** in the spleen. This process is a critical aspect of heme metabolism.

## Why the Correct Answer is Right
The correct answer, , indicates that about 80% of bilirubin is produced from the breakdown of old RBCs. This process involves the **splenic macrophages** engulfing and breaking down old RBCs, releasing heme, which is then converted into bilirubin through a series of enzyme-catalyzed reactions. Specifically, **heme oxygenase** catalyzes the conversion of heme to biliverdin, which is then converted to bilirubin by **biliverdin reductase**.

## Why Each Wrong Option is Incorrect
* **Option A:** - This option suggests that only 10% of bilirubin comes from old RBCs, which underestimates the contribution of RBC breakdown to bilirubin production.
* **Option B:** - This option implies that 50% of bilirubin production comes from old RBCs, which is lower than the actual percentage.
* **Option D:** - This option states that 90% of bilirubin comes from old RBCs, which slightly overestimates the actual contribution.

## Clinical Pearl / High-Yield Fact
A key point to remember is that the majority of bilirubin production comes from the **catabolism of red blood cells**, with a smaller fraction coming from other sources like **myoglobin** and **cytochromes**. Clinically, an increase in indirect (unconjugated) bilirubin can indicate **hemolysis** of red blood cells.

## Correct Answer Line
**Correct Answer: C. 80%**