Which tissue is the next most radiation-sensitive after bone marrow?

• A. CNS
• B. Heart
• C. GI tract
• D. Skin

Answer: (C)

Radiosensitivity tracks how actively a tissue’s cells are dividing and how mature those cells are. Bone marrow is the most sensitive because hematopoietic stem cells divide rapidly and continuously. The next most sensitive tissue is the GI tract, specifically the rapidly renewing epithelium of the intestinal crypts. These crypt cells turn over quickly to maintain the lining, so ionizing radiation readily damages them, leading to mucosal injury and symptoms like diarrhea and enteritis.

In contrast, the CNS is relatively resistant because neurons are largely non-dividing and mature; the heart muscle has low turnover, and the skin, while it does renew, has a slower rate of proliferation than the intestinal crypts. So the GI tract sits next in line after bone marrow in terms of radiation sensitivity.