What is a competitive inhibitor?

• A. A molecule that binds to the enzyme-substrate complex after binding has occurred.
• B. A molecule that binds to the enzyme's allosteric site, changing shape and reducing activity.
• C. A molecule that increases enzyme turnover by stabilizing the transition state.
• D. A molecule that binds to the enzyme's active site, blocking substrate binding and reducing reaction rate.

Answer: (D)