Which of the following is the hallmark histologic finding in the bone marrow of a patient with multiple myeloma?

Question

Accepted Answer

B. Increased plasma cells (>10%) with abnormal morphology. ## Bone Marrow Histology in Multiple Myeloma

**Key Point:** The hallmark histologic finding in multiple myeloma is increased bone marrow plasma cells (>10% of nucleated cells), often with abnormal morphology including immature features, binucleate forms, and increased mitotic activity.

### Diagnostic Criteria for Multiple Myeloma (IMWG 2014)

| Criterion | Threshold | Notes |
| --- | --- | --- |
| **Bone marrow plasma cells** | ≥10% | Hallmark finding; abnormal morphology supports diagnosis |
| **Serum monoclonal protein** | ≥3 g/dL | IgG, IgA, IgD, or IgM |
| **Urine monoclonal protein** | ≥500 mg/24 h | Bence Jones proteinuria |
| **CRAB criteria** | ≥1 present | Calcium elevation, Renal insufficiency, Anemia, Bone lesions |
| **Clonality** | Confirmed | Serum/urine electrophoresis + immunofixation |

### Morphologic Features of Abnormal Plasma Cells in MM

- **Immature cytology:** Irregular nuclear membranes, coarse chromatin, prominent nucleoli
- **Increased mitotic activity:** Abnormally high mitotic index
- **Binucleate/multinucleate forms:** Dysplastic features
- **Plasmacytosis:** Sheets or clusters replacing normal marrow elements
- **Cytoplasmic inclusions:** Russell bodies (immunoglobulin aggregates), Dutcher bodies (nuclear immunoglobulin)

**High-Yield:** A bone marrow plasma cell percentage ≥10% is **required** for the diagnosis of multiple myeloma. This is one of the three pillars of the diagnostic triad (along with serum/urine monoclonal protein and CRAB/SLiM criteria).

**Clinical Pearl:** The morphologic abnormality of plasma cells in MM can be graded by the **Matutes score** or similar systems; more immature morphology correlates with higher proliferation index and worse prognosis.

**Mnemonic:** **"10% Plasma = MM Diagnosis"** — Remember that ≥10% clonal bone marrow plasma cells is the morphologic threshold for MM diagnosis.

Answer

A. Sheets of mature lymphocytes with cleaved nuclei

Answer

C. Hemophagocytic histiocytes

Answer

D. Auer rods and myeloblasts

Which of the following is the hallmark histologic finding in the bone marrow of a patient with multiple myeloma?

Explanation

Bone Marrow Histology in Multiple Myeloma

Diagnostic Criteria for Multiple Myeloma (IMWG 2014)

Morphologic Features of Abnormal Plasma Cells in MM

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Criterion	Threshold	Notes
Bone marrow plasma cells	≥10%	Hallmark finding; abnormal morphology supports diagnosis
Serum monoclonal protein	≥3 g/dL	IgG, IgA, IgD, or IgM
Urine monoclonal protein	≥500 mg/24 h	Bence Jones proteinuria
CRAB criteria	≥1 present	Calcium elevation, Renal insufficiency, Anemia, Bone lesions
Clonality	Confirmed	Serum/urine electrophoresis + immunofixation