This is a real-world medical entrance exam question, please give the true answer based on the question and selection. Topic name: Inflammation & Repair, Subject name: Pathology
Mechanism of oxygen toxicity is? 
Select and explain. 
A. Direct DNA damage, B. Apoptosis, C. Nuclear fragmentation, D. Free radicals
D: Free radicals
Answer: D: Free radicals 
Explanation: ANSWER: (D) Free radicalsREF: Pulmonary physiology by Michael G. Levitzky page Page 245, Yao & Artusio's anesthesiology: problem-oriented patient management - Page 67"The mechanism of oxygen toxicity is controversial but probably involves the formation of superoxide anions or other free radicals""The biochemical basis for the toxicity of oxygen is the partial reduction of oxygen by one or two electrons to form reactive oxygen species"