This is a real-world medical entrance exam question, please give the true answer based on the question and selection. Topic name: Pharmacodynamics, Subject name: Pharmacology
Therapeutic index of a drug is a measure of its? 
Select and explain. 
A. Safety, B. Efficacy, C. Potency, D. Selectivity
A: Safety
Answer: A: Safety 
Explanation: Therapeutic Index tells about the safety margin of drug; narrow therapeutic index means unsafe drug. TI : LD50/ED50 *LD50 : lethal dose in 50% of population.*ED50 : effective dose in 50% of population or median effective dose.*LD50 and ED50 can be calculated by plotting a quantal DRC. * In this Quantal DRC, x-axis represents the log dose and y-axis represents the percentage of population responding.* Graded dose response curve tells about the potency and efficacy of a given drug in a paicular individual.