**Core Concept**
Polyostotic lesions refer to the presence of multiple bone lesions in various parts of the skeleton. This condition can be caused by various factors, including metabolic disorders, infections, and neoplastic processes. In the context of polyostotic lesions, it is essential to identify the underlying cause and differentiate it from other conditions that may present with similar clinical manifestations.

**Why the Correct Answer is Right**
Polyostotic lesions are commonly seen in conditions such as Paget's disease, multiple myeloma, and metastatic disease from other primary sites like breast or prostate cancer. However, in the case of **Fibrous Dysplasia (FD)**, polyostotic lesions are not typically seen. Fibrous dysplasia is a genetic disorder characterized by the replacement of normal bone with fibrous tissue, leading to weakened bones and potential deformities. While FD can be monostotic (affecting a single bone) or polyostotic (affecting multiple bones), the latter is relatively rare.

**Why Each Wrong Option is Incorrect**
**Option A:** This option is incorrect as Paget's disease can indeed present with polyostotic lesions, although the disease typically affects a single bone before spreading to other bones.
**Option B:** Multiple myeloma is a type of plasma cell dyscrasia that can cause polyostotic lesions due to the infiltration of malignant plasma cells into bone marrow and subsequent bone destruction.
**Option C:** Metastatic disease from other primary sites can cause polyostotic lesions, as cancer cells can spread to and affect multiple bones in the body.

**Clinical Pearl / High-Yield Fact**
When evaluating patients with polyostotic lesions, it is crucial to consider the underlying cause and differentiate between various conditions based on clinical presentation, imaging findings, and laboratory results. A thorough history and physical examination, combined with radiographic and laboratory studies, can help guide the diagnosis and management of these patients.

**Correct Answer:** D.