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

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

Answer: (D)

Radiosensitivity varies with how quickly cells divide. Lymphocytes are among the most radiosensitive cells in the body. They continually develop and divide in the bone marrow and lymphoid tissues, and their DNA is highly susceptible to radiation-induced damage. Because of this, lymphocytes show a rapid and substantial decline after irradiation, making them the most severely depressed bone-marrow element.

In contrast, lipocytes are relatively nondividing fat cells, neurons are highly resistant to radiation, and mature red blood cells are not actively dividing and are less immediately affected. Erythroid precursors can be impacted, but the immediate and most pronounced drop after irradiation is seen with lymphocytes.