A 32-year-old woman presents with pancytopenia, hepatosplenomegaly, and a bone marrow showing hypercellular marrow with dysplastic changes in all three cell lines. Which single feature best distinguishes myelodysplastic syndrome from aplastic anemia in this clinical context?

Explanation

## Distinguishing MDS from Aplastic Anemia ### Key Pathophysiologic Difference **Key Point:** Myelodysplastic syndrome (MDS) is characterized by **dysplasia in cell lines** within a **hypercellular or normocellular marrow**, whereas aplastic anemia presents with a **hypocellular marrow** devoid of dysplastic features. ### Comparison Table: MDS vs Aplastic Anemia | Feature | MDS | Aplastic Anemia | | --- | --- | --- | | **Bone marrow cellularity** | Hypercellular / normocellular | Hypocellular (< 25%) | | **Dysplasia** | Present in ≥1 cell line | Absent | | **Blasts** | 5–19% (by definition) | < 5% | | **Cytogenetic abnormalities** | Common (del 5q, trisomy 8, monosomy 7) | Absent | | **Hepatosplenomegaly** | Can occur (extramedullary hematopoiesis) | Rare | | **Transformation risk** | 25–30% to AML | < 5% | ### Why This Distinction Matters **High-Yield:** The **bone marrow morphology** is the gold standard discriminator: - **MDS**: Hypercellular marrow + dysplasia = ineffective hematopoiesis with risk of AML transformation. - **Aplastic anemia**: Hypocellular marrow + no dysplasia = quantitative failure of hematopoiesis, potentially reversible with immunosuppression. **Clinical Pearl:** Hepatosplenomegaly in MDS reflects extramedullary hematopoiesis attempting to compensate for marrow dysplasia; in aplastic anemia it is rare and suggests an alternative diagnosis (e.g., lymphoma, infection). ### Diagnostic Algorithm ```mermaid flowchart TD A[Pancytopenia]:::outcome --> B{Bone marrow cellularity?}:::decision B -->|Hypocellular| C[Aplastic Anemia]:::outcome B -->|Hypercellular/Normocellular| D{Dysplasia present?}:::decision D -->|Yes| E[MDS]:::outcome D -->|No| F[Other: TTP, SLE, etc.]:::outcome E --> G[Check cytogenetics + blasts]:::action C --> H[Assess for immune etiology]:::action ``` **Key Point:** Dysplasia in a hypercellular marrow is the **single best discriminator** between MDS and aplastic anemia.