**Core Concept**
Marble Bone disease, also known as Osteopetrosis, is a rare genetic disorder characterized by an abnormal increase in bone density due to a defect in osteoclast function. This results in fragile and brittle bones that are prone to fractures. The condition is caused by mutations in genes involved in osteoclast regulation, leading to impaired bone resorption.

**Why the Correct Answer is Right**
The correct answer is **C. Osteopetrosis**. Osteopetrosis is a result of defective osteoclast function, which is essential for bone resorption. Osteoclasts are multinucleated cells responsible for breaking down bone tissue, releasing minerals such as calcium and phosphorus into the bloodstream. In Marble Bone disease, the osteoclasts are defective, leading to an accumulation of dense, hard bone tissue.

**Why Each Wrong Option is Incorrect**
* **Option A:** Osteogenesis imperfecta is a different genetic disorder characterized by brittle bones and blue sclerae, but it is not Marble Bone disease.
* **Option B:** Paget's disease of bone is a condition that affects bone remodeling, but it is not Marble Bone disease.
* **Option D:** Fibrous dysplasia is a bone disorder characterized by fibrous tissue replacing normal bone, but it is not Marble Bone disease.

**Clinical Pearl / High-Yield Fact**
Marble Bone disease is often associated with an increased risk of osteoporotic fractures, particularly in the spine and hips. It is essential to recognize this condition early and manage it with medications that stimulate bone resorption.

**Correct Answer: C. Osteopetrosis**