This is a real-world medical entrance exam question, please give the true answer based on the question and selection. Topic name: Experimental Epidemiology, Subject name: Social & Preventive Medicine
Phase IV of Clinical trial is done: 
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: Ans. (d) Post marketing surveillanceRef.: K. Park 23rd ed. / 84-85Clinical TrialsPhaseSubjectsPurpose* Phase 1Healthy human volunteerSafety and non-toxicity* Phase 2Patients (20-200)Effectiveness* Phase 3PatientsComparison with existing drug* Phase 4PatientLong term side effects Post marketing surveillance* Most important phase of clinical trial: Phase 3* A new drug is launched in market after phase 3PHASE 4* Post marketing surveillance* Longest phase: 10-25 years* To study the long term side effects of a drug* Doesn't require ethical clearance.