Bone-marrow elements that are the most severely depressed after irradiation are:

• A. lipocytes
• B. neurons
• C. erythrocytes
• D. lymphocytes

Answer: (D)

Rapidly dividing cells are the most sensitive to ionizing radiation, so the bone marrow’s lymphoid lineage is hit hardest. Lymphocytes, whether developing in the marrow or circulating, have high proliferative activity and low differentiation at many stages, which makes them extremely radiosensitive. After irradiation, they decline quickly, leading to marked lymphopenia and immunosuppression. The other options are less affected: lipocytes are fat cells in the marrow that aren’t actively dividing; neurons are largely nondividing and fairly resistant to radiation; erythrocyte precursors can be affected, but their loss is not as immediate or profound as that of lymphocytes, and mature red cells aren’t proliferating in the marrow.