Many drugs are competitive inhibitors of specific enzymes. in such cases, the amount of drug needed to achieve a. the amount of drug needed to achieve the same effect is increased as the substrate concentration decreases. b. substrate must be bound to the enzyme before the drug can bind. c. the enzymes have to be allosterically regulated. d. the same effect is increased as the substrate concentration increases.

d. The amount of drug needed to achieve the same effect is increased as the substrate concentration increases.

Explanation:

A substrate binds to the active site of an enzyme, where it's transformed into a product. A competitive inhibitor is a molecule that has a similar structure to the natural substrate of an enzyme, and competes with it for the active site. If the competitive inhibitor is bound to the active site, there's no room for the actual substrate, so the enzyme is inhibited.

If the concentration of the substrate is much higher than the concentration of the drug, most of the enzymes will find a substrate molecule in the medium instead of a drug molecule, and the inhibition will not be very effective. In order to achieve the inhibition effect, the drug concentration needs to be increased, so that most of the enzymes' active sites are bound to a drug molecule.

d. organisms have cells with different shapes and functions.