**Core Concept**
The question is testing the student's knowledge of a rare genetic disorder characterized by defective bone metabolism, leading to increased bone turnover, and deposition of abnormal calcium phosphate crystals.

**Why the Correct Answer is Right**
The patient's symptoms, including progressive anemia, hepatosplenomegaly, and osteomyelitis of the jaw, are classic for **Fibrodysplasia Ossificans Progressiva (FOP)**, a rare genetic disorder caused by a mutation in the ACVR1 gene. The mutation leads to the formation of heterotopic ossification, where muscle and soft tissue is replaced by bone, resulting in the characteristic "chalky white deposits" seen on X-ray. The anemia and hepatosplenomegaly are likely due to the body's excessive inflammatory response and the subsequent release of cytokines.

**Why Each Wrong Option is Incorrect**
* **Option A:** This option is incorrect because the clinical presentation and X-ray findings do not match **Paget's disease of bone**, which is characterized by localized bone destruction followed by disorganized bone formation.
* **Option B:** This option is incorrect because **Osteogenesis imperfecta** is a genetic disorder characterized by brittle bones and skeletal deformities, but it does not typically present with progressive anemia and hepatosplenomegaly.
* **Option C:** This option is incorrect because **Multiple Myeloma** is a type of blood cancer that can cause bone lesions, but the X-ray findings in this case are more consistent with a rare genetic disorder.

**Clinical Pearl / High-Yield Fact**
Fibrodysplasia Ossificans Progressiva (FOP) is a rare genetic disorder with a prevalence of approximately 1 in 2 million births. It is characterized by the formation of heterotopic ossification, leading to the replacement of muscle and soft tissue with bone, resulting in progressive loss of mobility and function.

**Correct Answer: C. Multiple Myeloma**