This is a real-world medical entrance exam question, please give the true answer based on the question and selection. Topic name: FMGE 2019, Subject name: Psychiatry
Which of the following is not a formal thought disorder? 
Select and explain. 
A. Delusion, B. Neologism, C. Derailment, D. Tangentiality
A: Delusion
Answer: A: Delusion 
Explanation: Delusion is a disorder of content of thought. Neologism, derailment, and tangentiality are formal thought disorder.