A 62-year-old woman with diagnosed type 2 diabetes lived alone and did essentially nothing to manage her illness, including disregarding her physician's instructions. She was taken to her local emergency room (ER) with severe, multiple infected foot lesions, which yielded a variety of opportunistic microbes with a mixture of antibiotic susceptibilities. The physician decided to treat with systemic and topical antimicrobials. Which of the following antimicrobial agents must only be used topically?

Penicillin (d) is well-tolerated as is itraconazole (c). Vancomycin (e) is irritating and can result in phlebitis at the site of injection; nephrotoxicity of aminoglycosides is exacerbated by concomitant use of vancomycin but can be controlled by careful dosage. Gentamicin (b) and other aminoglycosides are ototoxic and nephrotoxic, but these adverse effects can usually be controlled by monitoring of serum concentration and careful dosage. Bacitracin (a) is highly nephrotoxic, so much so that it cannot be administered systemically. Because bacitracin is poorly absorbed, it can be used topically, providing local antibacterial activity but no systemic toxicity.