This is a real-world medical entrance exam question, please give the true answer based on the question and selection. Topic name: Epidemiology, Subject name: Social & Preventive Medicine
Phase 4 clinical trial is done as 
Select and explain. 
A. To find safety and toxicity, B. To compare with existing drugs, C. Pre marketing surveillance, D. Post marketing surveillance
D: Post marketing surveillance
Answer: D: Post marketing surveillance 
Explanation: The phases of clinical research are the steps in which scientists do experiments with a health intervention in an attempt to find enough evidence for a process that would be useful as a medical treatment.