The phrase 'crowding out of other blood elements' is described in the material as a feature of which condition?

• A. Aplastic anemia
• B. Leukemia
• C. Sickle cell anemia
• D. Polycythemia vera

Answer: (A)

The concept being tested is how abnormal cell growth in the bone marrow can occupy the space that normally supports the production of all blood elements. In leukemia, malignant white cells (blasts) proliferate rapidly and fill the marrow, crowding out the normal erythroid precursors, megakaryocytes, and other myeloid cells. This displacement leads to pancytopenia—reduced red cells causing anemia, fewer white cells causing infections, and fewer platelets causing bleeding tendencies. That “crowding out” description is a hallmark of marrow invasion by leukemia, which is why it’s the best fit for this statement.

Aplastic anemia, by contrast, involves failure or destruction of hematopoietic stem cells with marrow that becomes hypocellular and is often replaced by fat or fibrous tissue, not crowded by malignant elements. Sickle cell anemia centers on a defect in hemoglobin that leads to hemolysis and compensatory marrow changes, but it doesn’t describe crowding of normal elements by malignant cells. Polycythemia vera features overproduction of red blood cells with a hypercellular marrow, but the problem is not crowding out of other lineages and cytopenias are not the defining feature.