Which aminoglycoside is primarily used to treat Gram-negative infections such as urinary tract infections?

• A. Erythromycin
• B. Amoxicillin
• C. Gentamicin
• D. Doxycycline

Answer: (C)

Aminoglycosides are bactericidal drugs especially effective against aerobic Gram-negative bacteria. Gentamicin is the quintessential aminoglycoside used to treat serious Gram-negative infections, including urinary tract infections, because it achieves high concentrations in the urine and rapidly inhibits protein synthesis by binding to the 30S ribosomal subunit, causing misreading of the genetic code. Its activity is particularly relevant for Enterobacterales that cause UTIs, and in hospital settings it’s often paired with a beta-lactam to exploit synergistic cell-wall damage that enhances drug entry. Other drugs listed belong to different classes with different spectrums: erythromycin is a macrolide for Gram-positive organisms and atypicals, amoxicillin is a beta-lactam with broader oral use but not the primary choice for Gram-negative UTIs, and doxycycline is a tetracycline with limited urinary penetration and activity against many UTI pathogens. Gentamicin fits best for treating Gram-negative UTIs.