Which of the following is a commonly used penicillin for treating infections?

Ampicillin is a commonly used penicillin antibiotic that is effective in treating a variety of bacterial infections. It belongs to the penicillin group of drugs, making it suitable for targeting certain types of bacteria, particularly Gram-positive organisms, as well as some Gram-negative bacteria. Its broad-spectrum activity makes it a versatile choice in clinical settings, particularly for respiratory tract infections, urinary tract infections, and meningitis.

In contrast, cephalexin is a cephalosporin antibiotic, which is related but distinct from penicillin. Ciprofloxacin is a fluoroquinolone antibiotic effective against a different spectrum of pathogens, primarily Gram-negative bacteria. Vancomycin is a glycopeptide antibiotic primarily used for serious infections caused by Gram-positive bacteria, especially in cases related to penicillin resistance. Each of these alternatives serves specific purposes in antibiotic therapy but does not fall within the penicillin category like ampicillin does.