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
Not a defense mechanism: 
Select and explain. 
A. Derailment, B. Repression, C. Distoion, D. Undoing
A: Derailment
Answer: A: Derailment 
Explanation: 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.