**Core Concept**
Wilson's disease is a genetic disorder characterized by excessive accumulation of copper in the body, particularly in the liver, brain, and other vital organs. This accumulation leads to liver disease, neurological symptoms, and psychiatric problems. The condition is inherited in an autosomal recessive pattern, meaning that a person must inherit two defective genes, one from each parent.

**Why the Correct Answer is Right**
The defective gene responsible for Wilson's disease is the ATP7B gene, which encodes a copper-transporting P-type ATPase. This enzyme plays a crucial role in regulating the transport of copper from the liver to other parts of the body and in the excretion of excess copper in bile. Mutations in the ATP7B gene disrupt this process, leading to copper accumulation and the symptoms of Wilson's disease. The ATP7B gene is located on chromosome 13 and is essential for maintaining copper homeostasis in the body.

**Why Each Wrong Option is Incorrect**
* **Option A:** The HFE gene is associated with hereditary hemochromatosis, a different genetic disorder characterized by excessive iron accumulation in the body. While both conditions involve metal accumulation, they are distinct and involve different genes.
* **Option B:** The CCR5 gene is involved in the immune system and is associated with resistance to certain strains of HIV. It is not related to Wilson's disease.
* **Option D:** The CFTR gene is defective in cystic fibrosis, a genetic disorder that affects the respiratory and digestive systems. It is not associated with copper accumulation or Wilson's disease.

**Clinical Pearl / High-Yield Fact**
Wilson's disease can be treated with chelating agents such as penicillamine, which bind to excess copper and facilitate its excretion from the body. Patients with Wilson's disease require lifelong treatment and regular monitoring to prevent complications.

**Correct Answer:** C. ATP7B gene.