USMLE Step 3 Drug MOA Practice Test

Beta-lactam antibiotics, such as penicillin, exert their antimicrobial effect primarily through the activation of autolytic enzymes in bacteria. The mechanism involves the inhibition of transpeptidase, an enzyme critical for cross-linking peptidoglycan layers in bacterial cell walls. This leads to a weakening of the cell wall structure and triggers the activation of autolytic enzymes, which cause the bacteria to degrade their own cell wall, ultimately leading to cell lysis and death.

Penicillin binds to penicillin-binding proteins (PBPs), which are a group of proteins that include transpeptidases. When penicillin binds to these PBPs, it inhibits their enzymatic activity, disrupting the synthesis of the bacterial cell wall. As the cell wall becomes compromised, the autolytic enzymes are activated, further contributing to the lytic process.

Other options, while they describe important actions of various drugs, do not apply to beta-lactam antibiotics. For instance, inhibiting DNA synthesis pertains to antibiotics like fluoroquinolones, while inhibiting HMG-CoA reductase is relevant to statins, which are used for lowering cholesterol. Additionally, inhibiting dihydrofolate reductase describes the action of drugs such as