Which antibiotic listed is a macrolide used for Gram-positive ocular infections?

• A. Erythromycin
• B. Gentamicin
• C. Terramycin
• D. Ofloxacin

Answer: (A)

Macrolide antibiotics work by binding to the 50S ribosomal subunit and blocking translocation, which mainly curbs growth in Gram-positive bacteria that commonly cause ocular surface infections. Erythromycin is the classic macrolide formulated as an ophthalmic preparation, making it a preferred option for treating Gram-positive ocular infections in horses such as conjunctivitis caused by Staphylococcus or Streptococcus species.

The other drugs belong to different antibiotic classes: gentamicin is an aminoglycoside with strong Gram-negative activity (and some Gram-positives but not a macrolide), terramycin is a tetracycline with broad-spectrum activity, and ofloxacin is a fluoroquinolone with broad Gram-negative and some Gram-positive activity but not a macrolide.