Prevention is always better than Cure.

QA should try to prevent the defects from occuring. This 
can be achieved by 
1)Reviewing the Requirements
2)Reviewing the Design
3)Reviewing the Code
i.e., incorporating Quality assurance into all the phases 
of Development Life Cycle.

Phase containment (means to prevent any harmful substance 
from spread).If a defect is detected at design stage...it 
passes on to next stage if not corrected or fixed. In order 
to correct the defect one should track back the defect and 
correct that in the requirements and then in design...
Defect prevention-QA is mainly oriented towards defect 
orientation.