This is a real-world medical entrance exam question, please give the true answer based on the question and selection. Topic name: None, Subject name: Psychiatry
An elderly male patient on evaluation is found to have delusions, hallucinations and disturbed cognitive function. What is he most likely suffering from? 
Select and explain. 
A. Paranoid psychosis, B. Organic brain syndrome, C. OCD, D. Dissociative disorder
B: Organic brain syndrome
Answer: B: Organic brain syndrome 
Explanation: Organic brain syndromes refers to physical disorders that cause impaired mental function. It is the most common form of mental illness in elderly. In acute form it can present with delerium and in chronic form as dementia.