Which of the following is classified as an extended-spectrum penicillin?

The classification of extended-spectrum penicillins includes those that have been specifically designed to target a broader range of Gram-negative bacteria compared to traditional penicillins. Ticarcillin is recognized as an extended-spectrum penicillin because it can effectively combat certain Gram-negative organisms, including Pseudomonas aeruginosa, due to its broader antimicrobial activity.

Additionally, extended-spectrum penicillins like ticarcillin often have a more complex structure that allows them to penetrate the outer membrane of Gram-negative bacteria, thus providing a therapeutic advantage in treating infections caused by these organisms. The distinction of ticarcillin lies in its effectiveness against a larger spectrum of bacteria, primarily due to its ability to resist hydrolysis by certain types of bacterial β-lactamases.

In contrast, the other options do not fit the classification of extended-spectrum penicillins. Ceftriaxone is a cephalosporin antibiotic, azithromycin is a macrolide, and doxycycline is a tetracycline; each belongs to different classes of antibiotics with distinct mechanisms of action and spectra of activity.