A 82-year-old woman with a history of diabetes mellitus, coronary artery disease, and congestive heart failure is hospitalized for sepsis secondary to a urinary tract infection. She complains of disequilibrium and tinnitus which has developed over the past few days. In addition, she cannot hear high frequency sounds well. Which of the following of her medications is the most likely culprit?

Aspirin
Gentamicin
Hydrochlorothiazide
Metformin