**Core Concept**
Multiple myeloma is a plasma cell neoplasm characterized by monoclonal gammopathy, bone lesions, and marrow infiltration. Diagnosis relies on detecting abnormal immunoglobulins, bone marrow involvement, and evidence of skeletal damage.

**Why the Correct Answer is Right**
Bone scan detects bone turnover and areas of increased metabolic activity, which may show abnormal uptake in myeloma-related lesions. However, it is **non-specific**, has low sensitivity for early or small lesions, and cannot confirm the presence of plasma cell infiltration or detect monoclonal gammopathy. It is also less sensitive than imaging modalities like X-ray or MRI in early disease. In contrast, bone marrow biopsy is definitive for diagnosing plasma cell infiltration, and X-ray remains a key initial screening tool for osteolytic lesions.

**Why Each Wrong Option is Incorrect**
Option A: ESR (Erythrocyte Sedimentation Rate) is a nonspecific marker of inflammation and is often elevated in multiple myeloma due to monoclonal protein-induced inflammation. While not diagnostic, it provides supportive evidence and is clinically useful.
Option B: X-ray is the most accessible and widely used initial imaging tool in multiple myeloma, showing characteristic lytic bone lesions, especially in vertebrae and ribs. It remains a cornerstone in diagnosis and monitoring.
Option D: Bone marrow biopsy is the gold standard for diagnosing multiple myeloma, as it allows direct visualization of plasma cell infiltration and immunohistochemical confirmation.

**Clinical Pearl / High-Yield Fact**
Bone scan is **least specific** and **least useful** in early or subtle cases of multiple myeloma — it should not be relied upon as a primary diagnostic tool. Always use it in conjunction with more specific tests like X-ray, MRI, and bone marrow biopsy.

✓ Correct Answer: C. Bone scan