## **Core Concept**
Multiple myeloma is a type of hematological malignancy characterized by the proliferation of malignant plasma cells in the bone marrow. It leads to the destruction of bone tissue, resulting in lytic lesions. The disease often affects various bones in the body.

## **Why the Correct Answer is Right**
The correct answer, **D. Vertebral column**, is the commonest site for lytic lesions in multiple myeloma because the vertebral column, along with the skull, ribs, and pelvis, are rich in red marrow where plasma cells reside. The vertebral bodies are particularly prone to myeloma-related damage due to their high concentration of hematopoietic marrow and their trabecular bone structure, which is more susceptible to resorption.

## **Why Each Wrong Option is Incorrect**
- **Option A:** While the **femur** can be a site for lytic lesions, it is not the most common site compared to the vertebral column.
- **Option B:** The **humerus** can also be involved but is less frequently affected compared to the axial skeleton like the vertebral column.
- **Option C:** The **skull** is another common site for lytic lesions in multiple myeloma, often showing the characteristic "punched-out" lesions, but it is not more common than the vertebral column.

## **Clinical Pearl / High-Yield Fact**
A key clinical pearl is that multiple myeloma can cause vertebral collapse leading to back pain, and vertebral fractures are a common presenting feature. The disease's predilection for the vertebral column also explains why patients might experience symptoms like vertebral collapse or spinal cord compression.

## **Correct Answer:** D. Vertebral column.