One of the prominent third generation cephalosporins is not effective against which of the following pathogens?

Third generation cephalosporins are known for their broad spectrum of activity against various Gram-negative bacteria and some Gram-positive bacteria. In the context of the question, Staphylococcus aureus is primarily a Gram-positive bacterium, and while some third generation cephalosporins may have limited activity against it, they are generally not the first-line choice for treating infections caused by this pathogen.

In contrast, Haemophilus influenzae, Pseudomonas aeruginosa, and Enterobacter cloacae are typically susceptible to third generation cephalosporins. Haemophilus influenzae is a common organism treated with these antibiotics, as are Enterobacter cloacae and Pseudomonas aeruginosa, which may require specific agents like ceftazidime (a third generation cephalosporin) that target Pseudomonas specifically.

Thus, Staphylococcus aureus stands out as the pathogen against which prominent third generation cephalosporins are not effectively utilized, making it the correct answer in this context. This understanding emphasizes the importance of selecting appropriate antibiotics based on the specific microorganisms causing an infection.