**Core Concept**
Wilson disease is a genetic disorder caused by a defect in the ATP7B gene, leading to impaired copper transport and accumulation in the liver, brain, and other organs. This results in the toxic effects of copper on these tissues.

**Why the Correct Answer is Right**
The correct answer is copper, which accumulates in Wilson disease due to mutations in the ATP7B gene. The ATP7B protein is responsible for transporting copper into the Golgi apparatus for incorporation into ceruloplasmin, a copper-carrying protein in the blood. Without functional ATP7B, copper accumulates in the liver and other tissues, leading to cellular damage and disease.

**Why Each Wrong Option is Incorrect**
* **Option A:** Ceruloplasmin - While ceruloplasmin is a copper-carrying protein, Wilson disease is caused by a defect in copper transport, not accumulation of ceruloplasmin itself.
* **Option B:** Iron - Iron accumulation is associated with conditions such as hemochromatosis, not Wilson disease.
* **Option D:** Manganese - Manganese accumulation is associated with conditions such as manganism, not Wilson disease.

**Clinical Pearl / High-Yield Fact**
Wilson disease is an autosomal recessive disorder, meaning that individuals must inherit two defective copies of the ATP7B gene (one from each parent) to develop the disease.

**Correct Answer: C. Copper.**