Which of the following is a penicillin antibiotic?

• A. Amoxicillin (Amoxil)
• B. Ciprofloxacin
• C. Doxycycline
• D. Ceftriaxone (Rocephin)

Answer: (A)

Understanding which drugs are penicillin antibiotics hinges on recognizing the penicillin family and its mechanism. Amoxicillin is a penicillin antibiotic; it belongs to the beta-lactam class and works by binding to penicillin-binding proteins, preventing cross-linking of the bacterial cell wall, which leads to cell lysis. This mechanism is characteristic of penicillins, distinguishing it from the others listed. Ciprofloxacin is a fluoroquinolone that inhibits DNA gyrase and topoisomerase IV, not a beta-lactam. Doxycycline is a tetracycline that blocks the 30S ribosomal subunit to inhibit protein synthesis. Ceftriaxone is a cephalosporin, another beta-lactam class, but not a penicillin. Therefore, amoxicillin is the penicillin antibiotic.