NAPLEX Question of the Week: Antibiotic MOA

Some studies have found as many as 1 in 2 prescriptions written in the United States are for antimicrobials, often inappropriately. Today's question of the week involves mechanisms of action.

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A 68 year old female is admitted to the medical intensive care unit from the nursing home for septic shock likely due to pneumonia. The patient has NKDA and is written for vancomycin, piperacillin/tazobactam, and tobramycin for empiric therapy. Which of the following antibiotics that  she is receiving exerts its mechanism of action by inhibiting cell wall synthesis? 

A. Vancomycin and piperacillin/tazobactam

B. Piperacillin/tazobactam and tobramycin

C. Vancomycin and tobramycin

D. Vancomycin

E. Tobramycin

Answer with rationale:

The correct answer is A. All beta-lactams, carbapenems, and monobactams (remember aztreonam?) decrease cell wall synthesis leading to cell death in the bacteria targeted. Remember that beta-lactams include penicillins and cephalosporins. Vancomycin works by binding to the D-alanyl-D-alanine terminal of the growing peptide chain during cell wall synthesis, resulting in inhibition of transpeptidase, which prevents further elongation and cross-linking of the peptidoglycan matrix. Aminoglycosides such as tobramycin work to inhibit protein synthesis by irreversible binding to the 30S ribosomal subunit making answers B, C, and E incorrect. 

Mechanisms of action are crucial to foundational knowledge to understand clinical pharmacotherapy. By understanding mechanism of action, you further solidify pathophysiology by understanding drug targets for disease and don't "memorize" material. Knowledge of mechanisms of action falls into NAPLEX competency statement 1.2.5. 

Have a great week!

Dr. B

Christopher M. Bland

Clinical Professor, University of Georgia College of Pharmacy

Dr. Christopher M. Bland is a Clinical Professor at the University of Georgia College of Pharmacy at the Southeast GA campus in Savannah, GA. Dr. Bland has over 20 years of academic and clinical experience in a number of clinical areas. He is a Fellow of both the Infectious Diseases Society of America as well as the American College of Clinical Pharmacy. He is co-founder of the Southeastern Research Group Endeavor, SERGE-45, with over 80 practitioners across 14 states involved. Dr. Bland serves as Associate Editor for the NAPLEX Review Guide 4th edition as well as Editor-In-Chief for the Question of the Week. He has provided live, interactive reviews for more than 10 Colleges/Schools of Pharmacy over the course of his career.