Which of the following antimicrobials would NOT be effective against beta-lactamase producing Moraxella catarrhalis?

Amoxicillin would not be effective against beta-lactamase producing Moraxella catarrhalis due to the presence of beta-lactamase enzymes produced by the bacteria. These enzymes can hydrolyze and inactivate beta-lactam antibiotics, including penicillins like amoxicillin. In contrast, the other antimicrobials listed, such as ciprofloxacin, azithromycin, and cephalexin, are not affected by these enzymes in the same way.

Ciprofloxacin is a fluoroquinolone that works through a different mechanism, targeting bacterial DNA gyrase and topoisomerase IV, and it remains effective even when the bacteria produce beta-lactamases. Azithromycin, a macrolide, also functions through a separate mechanism, inhibiting bacterial protein synthesis by binding to the 50S ribosomal subunit. Cephalexin, despite being a cephalosporin, is less commonly affected by the specific types of beta-lactamases produced by Moraxella catarrhalis when compared to amoxicillin.

Therefore, amoxicillin is the antibiotic that would typically be rendered ineffective in the presence of these enzymes, making it the correct choice.