**Core Concept**
The question is testing the knowledge of green stains that are commonly seen in children, which is related to the field of pediatric dermatology. These stains are often caused by the accumulation of certain substances in the skin, leading to discoloration.

**Why the Correct Answer is Right**
The correct answer is due to the presence of **bilirubin**, a yellow compound that is a byproduct of the breakdown of red blood cells. In children, the liver is still maturing and may not be able to process bilirubin efficiently, leading to its accumulation in the skin. This results in a greenish discoloration, often seen on the palms and soles of the feet. The green coloration is due to the presence of **biliverdin**, a green pigment that is also a byproduct of bilirubin breakdown.

**Why Each Wrong Option is Incorrect**
* **Option A:** This option is incorrect because it does not specify a known cause of green stains in children.
* **Option B:** This option is incorrect because it mentions a substance that is not typically associated with green stains in children.
* **Option C:** This option is incorrect because it is a type of skin condition that is not typically characterized by green stains.

**Clinical Pearl / High-Yield Fact**
It's essential to remember that jaundice, which is characterized by yellowing of the skin and eyes, can sometimes appear greenish in children due to the presence of biliverdin. This is often seen in infants and young children with hemolytic disease or liver dysfunction.

**Correct Answer:** C. Bilirubin.