Therapeutic choices for Pseudomonas aeruginosa — recognizing ineffective agents Which antibiotic below is generally not clinically useful against Pseudomonas aeruginosa infections?

Introduction / Context:
Pseudomonas aeruginosa is a non-fermenting gram-negative bacillus with intrinsic and acquired resistance mechanisms. Selecting an agent with reliable antipseudomonal activity is crucial in severe infections such as ventilator-associated pneumonia, bacteremia, and complicated urinary tract infection.

Given Data / Assumptions:

• The question asks for an agent generally not useful against P. aeruginosa.
• Common antipseudomonal classes include specific beta-lactams, aminoglycosides, and fluoroquinolones.
• Trimethoprim–sulfamethoxazole (TMP–SMX) has poor, unreliable activity against P. aeruginosa.

Concept / Approach:
Active options typically include piperacillin (especially with tazobactam), ceftazidime or cefepime, carbapenems (meropenem, imipenem except ertapenem), aztreonam, and aminoglycosides (tobramycin, amikacin). TMP–SMX targets folate pathways but is not a dependable choice for P. aeruginosa due to intrinsic resistance and efflux mechanisms.

Step-by-Step Solution:
Evaluate each option's spectrum: aminoglycosides active (often in combination); ceftazidime is antipseudomonal; piperacillin has activity (enhanced with tazobactam).Confirm TMP–SMX lacks reliable efficacy against P. aeruginosa.Select Trimethoprim–sulfamethoxazole as the agent not clinically useful.

Verification / Alternative check:
Institutional antibiograms and guidelines rarely list TMP–SMX for Pseudomonas; susceptibility rates are typically very low compared to antipseudomonal beta-lactams and aminoglycosides.

Why Other Options Are Wrong:
Aminoglycosides, ceftazidime, and piperacillin show recognized antipseudomonal activity, especially in combination therapy for severe disease. Piperacillin–tazobactam is also widely used.

Common Pitfalls:
Assuming broad gram-negative activity implies antipseudomonal activity; many agents lack Pseudomonas coverage despite gram-negative spectra.

Final Answer:
Trimethoprim–sulfamethoxazole.