This is a real-world medical entrance exam question, please give the true answer based on the question and selection. Topic name: General Pharmacology, Subject name: Pharmacology
Which phase of clinical trials is done after the drug enters the market? 
Select and explain. 
A. Phase I, B. Phase II, C. Phase III, D. Phase IV
D: Phase IV
Answer: D: Phase IV 
Explanation: Ans. (D) Phase IV(Ref: KDT 8th/e 91)Clinical TrialOther NamePhase 0Microdosing studiesPhase IHuman pharmacology and safetyPhase IITherapeutic exploratoryPhase IIITherapeutic confirmatoryPhase IVPost marketing surveillance