A patient presents with evidence of lytic lesion in the bone. Histology from the bone lesion reveals monoclonal plasma cells. Bone marrow biopsy from the from the sternum shows <10 percent Plasma cells. Skeletal Survey is otherwise unremarkable. A small positive M spike is repoed on electrophoresis. The diagnosis is:

**Core Concept**
The patient presents with a lytic bone lesion and histological evidence of monoclonal plasma cells, but the diagnostic criteria for multiple myeloma are not fully met due to the low percentage of plasma cells in the bone marrow and the absence of significant end-organ damage. This clinical scenario highlights the importance of distinguishing between multiple myeloma and other plasma cell disorders.

**Why the Correct Answer is Right**
The presence of a monoclonal spike on electrophoresis and a lytic bone lesion suggests a plasma cell disorder. However, the low percentage of plasma cells in the bone marrow (<10%) and the lack of significant end-organ damage (e.g., renal impairment, anemia, or hypercalcemia) make multiple myeloma less likely. Instead, this patient's findings are more consistent with a diagnosis of monoclonal gammopathy of undetermined significance (MGUS), which is characterized by the presence of a monoclonal protein spike without significant end-organ damage or a high percentage of plasma cells in the bone marrow. **Why Each Wrong Option is Incorrect** **Option A:** Multiple myeloma is unlikely due to the low percentage of plasma cells in the bone marrow and the absence of significant end-organ damage. **Option B:** Plasmacytoma is typically characterized by a solitary bone lesion and a higher percentage of plasma cells in the bone marrow. **Option C:** Waldenström macroglobulinemia is a type of lymphoma that typically presents with a higher percentage of lymphoplasmacytic cells in the bone marrow and a significant monoclonal spike on electrophoresis. **Clinical Pearl / High-Yield Fact** MGUS is a common condition that can precede multiple myeloma and other plasma cell disorders. It is essential to monitor patients with MGUS for signs of progression to multiple myeloma or other complications. **Correct Answer:** Monoclonal Gammopathy of Undetermined Significance (MGUS)

✓ Correct Answer: B. Plasmacytoma