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Competitive inhibitor is when the inhibitor which closely resembles the substrate inhibits the activity of the enzyme. The inhibitor resmbles the substrate in its molecular structure and therefore competes with the substrate for the substrate binding site. As a result the substrate cannot bind to the site and the enzyme action declines. 
For example - ihibition of substrate dehydrogenase by malonate. In this malonate resembles the substrate succinic acid in structure.