Final answer:
About half of the medical drugs currently in use share the general action of inhibiting enzyme activity(option a).
Step-by-step explanation:
The general action shared by about half of the medical drugs currently in use is a) Inhibition of enzyme activity.
Many drugs act as enzyme inhibitors, interfering with the catalytic action of enzymes and thus altering physiological processes. In some cases, drugs that inhibit enzymes can lead to the decrease in the synthesis of certain enzymes, the alteration of cell reactions, and the inactivation of enzymes. This is exemplified by drugs that inhibit adenylyl cyclase, blocking the synthesis of cAMP in the G-protein signaling pathway, which subsequently affects action through a second messenger and various metabolic pathways.