**Core Concept**
Pink's disease is a rare condition characterized by acrodynia or pink disease, primarily affecting children. The underlying cause is the toxicity of a specific substance, leading to various systemic manifestations. This condition highlights the importance of recognizing the effects of environmental toxins on human health.

**Why the Correct Answer is Right**
The correct answer is mercury. Pink's disease is caused by the toxicity of methylmercury, a form of mercury that accumulates in the body and affects multiple systems. Methylmercury is a potent neurotoxin that damages the nervous system, causing symptoms such as pain, swelling, and discoloration of the extremities. The condition is often associated with exposure to mercury-containing products, such as skin ointments or teething powders, in infants and young children.

**Why Each Wrong Option is Incorrect**
**Option A:** This option is incorrect because it does not specify a known cause of Pink's disease. While arsenic is a toxic substance, it is not associated with Pink's disease.
**Option B:** This option is incorrect because it is a treatment for Pink's disease, rather than a cause. Chelation therapy is used to remove mercury from the body and alleviate symptoms.
**Option C:** This option is incorrect because it is not a known substance associated with Pink's disease. While certain heavy metals can cause toxicity, they are not the primary cause of Pink's disease.

**Clinical Pearl / High-Yield Fact**
It is essential to recognize the potential for environmental toxins to cause disease in children, particularly those exposed to mercury-containing products. Healthcare professionals should be aware of the risks associated with these substances and take steps to prevent exposure.

**Correct Answer:** C. Mercury.