**Question:** All of the following cephalosporins are active against Pseudomonas except:

A. Cephalexin
B. Cefazolin
C. Ceftriaxone
D. Cefotaxime

**Core Concept:** Cephalosporins are a class of drugs used for treating bacterial infections. They are beta-lactam antibiotics that inhibit bacterial cell wall synthesis by binding to penicillin-binding proteins (PBPs) in the bacteria. Pseudomonas is a genus of gram-negative bacteria that can cause infections in humans. Pseudomonas is known to be resistant to certain beta-lactam antibiotics due to the presence of beta-lactamases, which are enzymes that degrade beta-lactam antibiotics, making them ineffective.

**Why the Correct Answer is Right:** Cephalexin is a first-generation cephalosporin with a low molecular weight, making it susceptible to hydrolysis by beta-lactamases produced by Pseudomonas species. Cephalexin has a narrow spectrum of activity and is not active against gram-negative bacteria like Pseudomonas.

**Why Each Wrong Option is Incorrect:**

Cefazolin is a second-generation cephalosporin with a higher molecular weight than cephalexin, which makes it more resistant to beta-lactamases produced by Pseudomonas. While cefazolin is active against some gram-negative bacteria, it is not specific for Pseudomonas.

Ceftriaxone is a third-generation cephalosporin with a higher molecular weight than cephalexin and cefazolin, making it more resistant to beta-lactamases produced by Pseudomonas. Ceftriaxone is active against a broad range of bacteria, including some gram-negative bacteria like Pseudomonas, but is not specific for Pseudomonas.

Cefotaxime is a third-generation cephalosporin with a higher molecular weight than cephalexin and cefazolin, making it more resistant to beta-lactamases produced by Pseudomonas. Cefotaxime is active against a broad range of bacteria, including some gram-negative bacteria like Pseudomonas, but is not specific for Pseudomonas.

**Clinical Pearl:** Pseudomonas species are often resistant to beta-lactam antibiotics due to the production of beta-lactamases. To combat this resistance, newer beta-lactam antibiotics, like the cephalosporins, have been developed with higher molecular weights and extended spectra to include some gram-negative bacteria like Pseudomonas. However, these antibiotics are not specific for Pseudomonas and may not be ideal for treating Pseudomonas infections.