**Question:** Antimicrobial agent effective against ESBL producing microorganisms is

A. Ceftazidime
B. Cefotaxime
C. Aztreonam
D. Meropenem

**Correct Answer:** C. Aztreonam

**Core Concept:** Extended-spectrum beta-lactamases (ESBLs) are a type of enzyme produced by certain bacteria, such as Escherichia coli and Klebsiella pneumoniae, that can inactivate beta-lactam antibiotics like penicillins, cephalosporins, and monobactams, making them less effective or ineffective.

**Why the Correct Answer is Right:** Aztreonam is a monobactam, a type of beta-lactam antibiotic, that is resistant to ESBLs due to its structural modification. Unlike other beta-lactam antibiotics, it lacks the side chain that is commonly targeted by ESBLs, allowing it to inhibit bacterial cell wall synthesis and kill the bacteria.

**Why Each Wrong Option is Incorrect:**

A. Ceftazidime: Ceftazidime is a third-generation cephalosporin that can be inactivated by ESBLs, rendering it less effective against ESBL-producing bacteria.
B. Cefotaxime: Similar to ceftazidime, cefotaxime is a third-generation cephalosporin that can be inactivated by ESBLs, making it ineffective against ESBL-producing bacteria.
D. Meropenem: Meropenem is a carbapenem antibiotic, which is less susceptible to inactivation by ESBLs. However, it is still susceptible to inactivation by other mechanisms like AmpC beta-lactamases and porin loss.

**Clinical Pearl:** ESBL-producing bacteria are a common issue in healthcare settings, causing infections that may be challenging to treat with traditional beta-lactam antibiotics. Susceptibility testing and knowledge of antibiotic inactivation mechanisms by ESBLs are crucial for appropriate antibiotic choice and patient management.