This is a real-world medical entrance exam question, please give the true answer based on the question and selection. Topic name: Basics of Psychiatry, Subject name: Psychiatry
A 30-year-old woman mentions that the trunk of a paicular tree near her apament looks like face of lord Ram She can see the face of lord Ram every time when she walks past that tree but not on the other trees. This phenomenon is known as? 
Select and explain. 
A. Delusional misidentification, B. Delusion of reference, C. Illusion, D. Visual hallucination
C: Illusion
Answer: C: Illusion 
Explanation: An illusion is an involuntary false perception (i.e. face of Lord Ram) in which a transformation of real object (i.e. a specific tree in this case) takes place. Whereas hallucination is a disorder of perception in absence of a real stimulus.