**Core Concept**
Multiple myeloma is a malignant proliferation of plasma cells in the bone marrow, leading to destructive bone lesions. The characteristic "punched out" appearance on X-ray is due to the replacement of normal bone tissue with neoplastic plasma cells.

**Why the Correct Answer is Right**
The presence of multiple lytic lesions in the vertebrae, along with punched out lesions in the skull, is highly suggestive of multiple myeloma. This is because myeloma cells disrupt the normal bone remodeling process, resulting in areas of bone destruction without significant sclerosis. The lack of reactive bone formation is due to the inhibition of osteoblasts by myeloma cells and their products. Furthermore, the diffuse nature of myeloma bone disease involves the entire bone, rather than focal lesions seen in metastatic disease.

**Why Each Wrong Option is Incorrect**
**Option A:** Metastasis typically presents with mixed lytic and sclerotic lesions, as the tumor cells stimulate osteoblasts to form reactive bone.

**Option C:** Osteomalacia is characterized by a softening of the bones due to impaired mineralization, often due to vitamin D deficiency. This results in a ground-glass appearance on X-ray, not punched out lesions.

**Option D:** Hyperparathyroidism can cause brown tumors and generalized osteopenia, but the lesions are typically well-defined and not punched out.

**Clinical Pearl / High-Yield Fact**
Multiple myeloma can be diagnosed with a serum protein electrophoresis (SPEP) showing a monoclonal spike, which is a characteristic feature of this disease.

**✓ Correct Answer: B. Multiple myeloma**