A 58-year-old woman presents with back pain, anemia, and elevated serum creatinine. Bone marrow biopsy shows 40% plasma cells. Which is the most common site of lytic bone lesions in multiple myeloma?

Explanation

## Sites of Lytic Bone Lesions in Multiple Myeloma **Key Point:** The **skull and vertebral column** are the most common sites of lytic bone lesions in multiple myeloma, followed by the pelvis and ribs. ### Distribution of Bone Lesions | Site | Frequency | Clinical Significance | |------|-----------|----------------------| | **Skull** | ~50% | Characteristic "punched-out" lesions; easily visible on skull X-ray | | **Vertebral column** | ~40% | Vertebral compression fractures; spinal cord compression risk | | **Pelvis** | ~30% | Often asymptomatic; detected on imaging | | **Ribs** | ~25% | Pathological fractures common | | **Femur** | ~15% | Less common than axial skeleton | | **Humerus** | ~10% | Least common long bone involvement | ### Pathophysiology of Bone Involvement ```mermaid flowchart TD A[Myeloma cells in bone marrow]:::outcome --> B[RANKL activation<br/>Osteoclast stimulation]:::action A --> C[Osteoprotegerin inhibition<br/>Reduced osteoblast activity]:::action B --> D[Increased bone resorption]:::outcome C --> D D --> E[Lytic lesions]:::outcome E --> F[Hypercalcemia]:::urgent E --> G[Pathological fractures]:::urgent ``` **High-Yield:** Myeloma preferentially affects the **axial skeleton** (skull, spine, pelvis, ribs) because these sites have active bone marrow. Long bones of the extremities are less commonly involved because they contain fatty marrow in adults. **Clinical Pearl:** The characteristic "punched-out" lytic lesions on skull X-ray are pathognomonic for myeloma and are one of the diagnostic criteria. Vertebral involvement often presents with back pain and risk of spinal cord compression requiring urgent imaging (MRI). ### Why Axial Skeleton? The axial skeleton contains red (hematopoietic) marrow throughout life, whereas long bones transition to yellow (fatty) marrow in adulthood. Myeloma cells preferentially proliferate in areas of active marrow, explaining the predilection for skull, spine, and pelvis.