Answer:
The active functional group of a drug is that one that has the most prominent quality of adherence to the corresponding group on the target site and this selective binding of the group gave the property of targeted therapy and action. The pharmacophore modelling is done to ensure the design success of a drug but real life circumstances are mostly unpredictable. By this specific targeted action a drug acquires the property of differentiating between healthy and infected cells. Infected cells always have something different in their functional group expression as compare the the healthy cells. This helps the drug to differentiate between them.