Which statement describes an indirect effect of ionizing radiation?

• A. interaction with water generating free radicals
• B. direct interaction with DNA
• C. affects only external tissues
• D. none

Answer: (A)

Indirect effects come from radiation hitting water inside the cell and causing radiolysis, which creates reactive free radicals such as hydroxyl radicals and hydrogen atoms. These radicals then go on to damage critical biomolecules like DNA, proteins, and membranes. Because cells are mostly water, most radiation damage actually arises this way, making it the predominant pathway. In contrast, a direct interaction would mean the radiation ionizes DNA itself without the intermediary radicals. The idea that only external tissues are affected isn’t accurate, since the indirect pathway operates wherever water-containing tissues are exposed. So describing damage as resulting from water radiolysis and subsequent free radicals best captures an indirect effect.