Antibacterials (ABX) Practice Exam

The key idea is selecting an agent with strong activity against anaerobic bacteria, since those organisms are commonly involved in infections of the abdomen and pelvis. Metronidazole is highly effective against anaerobes, including the Bacteroides fragilis group and other anaerobic pathogens often found in intra-abdominal and gynecologic infections. It works by a nitroimidazole mechanism that is activated in anaerobic conditions, leading to DNA damage and bacterial death. It also has good tissue penetration and can be taken orally or given IV, making it a practical choice for treating these polymicrobial infections. In practice, metronidazole is often used to provide the anaerobic coverage in regimens for intra-abdominal and gynecologic infections, typically paired with another agent that covers aerobes for broad-spectrum therapy.

Doxycycline, while broad in some areas, does not reliably cover anaerobes in these contexts. Aztreonam and gentamicin primarily target aerobic Gram-negative bacteria and lack meaningful anaerobic coverage, so they’re not the best standalone options for infections where anaerobes are expected.