Final answer:
A competitive inhibitor affects both the Km and the initial rate of an enzymatic reaction but does not change the Vmax, making option 5) both choices A and B the correct answer.
Step-by-step explanation:
A competitive inhibitor will affect the kinetic parameters of an enzymatic reaction, specifically the Km (Michaelis-Menten constant) and potentially the apparent Vmax (maximum reaction velocity). Competitive inhibitors resemble the substrate and compete with it for binding to the enzyme's active site.
While competitive inhibition increases the apparent Km because a higher substrate concentration is needed to reach half of the Vmax, it does not change the actual Vmax. This is because the inhibitor can be outcompeted by an increasing concentration of substrate, allowing for the maximum rate of the enzymatic reaction to be reached.
In summary, a competitive inhibitor affects the Km and can also impact the initial rate but does not change the Vmax when substrate concentration is sufficiently high.
Answering the specific question, a competitive inhibitor will affect both the Km and the initial rate of an enzymatic reaction. Therefore, the correct option is 5) both choices A and B.