Question

A $54$-year-old woman with a history of type $2$ diabetes mellitus is evaluated in the hospital on day $3$ of admission for a liver abscess. She originally presented to the emergency department with a $4$-day history of right upper quadrant abdominal pain, high fever, and chills. At that time, an abdominal ultrasound revealed a $6\text{ cm}$ fluid collection in the right hepatic lobe. She underwent successful image-guided percutaneous drain placement and was empirically started on intravenous vancomycin, piperacillin-tazobactam, and metronidazole. Today, the patient reports significant improvement in her abdominal pain. Her temperature is $37.1^\circ\text{C}$ ($98.8^\circ\text{F}$), blood pressure is $124/78\text{ mm Hg}$, and pulse is $76/\text{min}$. The percutaneous drain has minimal purulent output. Both blood cultures and the initial abscess aspirate cultures (aerobic and anaerobic) are now growing pure *Klebsiella pneumoniae*. Susceptibility testing demonstrates the isolate is sensitive to ceftriaxone, cefepime, ciprofloxacin, and piperacillin-tazobactam, and is resistant to ampicillin. Which of the following is the most appropriate next step in pharmacotherapy?