A 30-year-old presents with pallor, fatigue, pancytopenia, and a hypocellular bone marrow. What is the most likely diagnosis?

• A. Acute leukemia
• B. Myelodysplastic syndrome
• C. Hairy cell leukemia
• D. Aplastic anemia

Answer: (D)

Pancytopenia with a markedly hypocellular bone marrow points to bone marrow failure from aplastic anemia. In aplastic anemia the hematopoietic stem cells are damaged or destroyed, so production of all blood lineages drops. The marrow becomes filled with fat and very few hematopoietic cells, which explains the low red cells, white cells, and platelets seen in the blood. A low reticulocyte count supports this, since the marrow isn’t actively producing new cells.

This pattern helps distinguish it from other causes. Acute leukemia usually shows a hypercellular marrow with many blasts and impaired maturation but not fat replacement. Hairy cell leukemia can cause cytopenias as well, but the marrow is typically infiltrated with hairy cells and often features splenomegaly. Myelodysplastic syndromes can cause cytopenias, but they more commonly present with dysplastic changes and may have a hypercellular marrow, especially in younger patients. The combination of severe pancytopenia with a hypocellular marrow and fatty replacement is classic for aplastic anemia.