**Core Concept**
Dimercaprol, also known as British Anti-Lewisite (BAL), is a chelating agent used in the treatment of heavy metal poisoning, such as arsenic, mercury, and lead. It works by binding to the metal ions, forming a stable complex that can be excreted from the body. However, its use is contraindicated in certain types of poisoning.

**Why the Correct Answer is Right**
The correct answer is related to the fact that dimercaprol can increase the toxicity of certain metals, particularly those that it cannot effectively chelate. This is crucial in understanding the limitations of dimercaprol therapy.

**Why Each Wrong Option is Incorrect**
**Option A:** This option might be considered incorrect because dimercaprol is often used in the treatment of arsenic poisoning.
**Option B:** Similarly, this option could be wrong as dimercaprol can be used for mercury poisoning under certain conditions.
**Option C:** This option is also incorrect as lead poisoning is another condition where dimercaprol might be considered, although it's not the first line of treatment.

**Why Each Wrong Option is Incorrect (Continued)**
Given the lack of specific details for each option, a general understanding of dimercaprol's use in heavy metal poisoning suggests that it would not be contraindicated in the poisoning types typically treated with it.

**Clinical Pearl / High-Yield Fact**
A key point to remember is that dimercaprol is contraindicated in iron poisoning because it can form a complex with iron that is more toxic than the iron itself.

**Correct Answer:** Correct Answer: D. Iron poisoning.