In one of the curves in the graph, the rate of an enzyme-catalysed reaction has been plotted against the substrate concentration in presence of a small quantity of a competitive inhibitor. Which curve represents competitive inhibition?

B

This question had very high discrimination; good candidates were able to answer it well. It is true that the question ought to have clearly stated that A was the graph showing enzyme activity without inhibitor, therefore allowing candidates to compare with the activity using an inhibitor. As this graph is in most books, candidates did not seem to have any problems answering that B was the curve showing competitive inhibition.