This is a real-world medical entrance exam question, please give the true answer based on the question and selection. Topic name: Psychoanalysis, Subject name: Psychiatry
[{'content': 'Not a defense mechanism: \nSelect and explain. \nA. Derailment, B. Repression, C. Distoion, D. Undoing', 'role': 'user'}, {'content': 'Answer: A \nExplanation: Derailment is a formal thought disorder and not a defense mechanism. Distoion involves modification of reality. Denial means refusal to accept the reality Undoing is an act to nullify the previous act.', 'role': 'assistant'}]