What is the five-step chain of events in radiation injury?

• A. Photon emission -> Tissue heating -> Pain -> Recovery -> Healing.
• B. Photon -> Scattering -> Absorption -> Emission -> Damage.
• C. Absorption -> Scattering -> Reflection -> Imaging -> Diagnosis.
• D. Photon -> Ionization -> Chemical Change -> Cellular Response -> Clinical Effect.

Answer: (D)

Radiation injury begins when a photon interacts with tissue and deposits energy, causing ionization. Ionization disrupts molecular bonds and often produces reactive species, especially from radiolysis of water, which leads to chemical changes in key biomolecules like DNA. Those chemical alterations trigger cellular responses—damage signaling, repair attempts, cell cycle arrest, or cell death through apoptosis or necrosis. The accumulation of these cellular effects manifests as clinical tissue injury or functional impairment.

Other sequences mix in processes like heating, scattering, or imaging terminology, or include steps (like emission) that don’t accurately describe how radiation injury develops. The injury pathway is driven by energy transfer leading to ionization, then chemical change, then cellular response, and finally clinical effect.