A 62-year-old woman is evaluated in the hospital on day 4 of admission. She initially pres… — USMLE Step 2 CK

A 62-year-old woman is evaluated in the hospital on day 4 of admission. She initially presented to the emergency department with a 2-day history of high fever, chills, and severe right-sided back pain. On admission, her temperature was $39.2^\circ\text{C}$ ( $102.6^\circ\text{F}$ ), blood pressure was $110/70\text{ mmHg}$ , and heart rate was $115\text{/min}$ . Physical examination revealed pronounced right costovertebral angle tenderness. Laboratory studies showed a leukocyte count of $16,500\text{/mm}^3$ . She was started on empiric intravenous ceftriaxone.

On hospital day 2, her fever persisted. Urine culture grew $>10^5\text{ CFU/mL}$ of *Escherichia coli*. The isolate was identified as an extended-spectrum beta-lactamase (ESBL) producer. Antimicrobial susceptibility testing showed the following:

- Ceftriaxone: Resistant
- Cefepime: Resistant
- Piperacillin-tazobactam: Resistant
- Meropenem: Susceptible
- Ertapenem: Susceptible
- Ciprofloxacin: Susceptible
- Nitrofurantoin: Susceptible
- Fosfomycin: Susceptible

Her regimen was subsequently changed to intravenous ertapenem. Today (hospital day 4), the patient is afebrile, her flank pain has resolved, and her leukocyte count has normalized to $8,500\text{/mm}^3$ . She is tolerating a regular diet and is eager to be discharged.

Which of the following is the most appropriate antimicrobial regimen for discharge?

Oral ciprofloxacinAnswer
B
Oral nitrofurantoin
C
Oral fosfomycin
D
Intravenous ertapenem via a peripherally inserted central catheter
E
Oral cefdinir

Answer

The most appropriate regimen for discharge is oral ciprofloxacin.

The patient has acute pyelonephritis caused by an ESBL-producing E. coli. She has clinically improved on intravenous ertapenem and is ready for discharge. The ideal step-down therapy should be an oral agent that is active against the isolate and achieves high concentrations in the renal parenchyma. Fluoroquinolones, such as ciprofloxacin, have excellent oral bioavailability and tissue penetration. Because the susceptibility report explicitly shows the isolate is susceptible to ciprofloxacin, it is the safest and most appropriate oral option to complete her treatment course.

Step-by-Step Solution

Diagnose the clinical condition and severity.

The patient presented with fever, flank pain, CVA tenderness, and leukocytosis, consistent with acute complicated pyelonephritis. She has now clinically resolved on intravenous therapy.

Establishing the diagnosis determines the required tissue penetration for the chosen antibiotic.

Analyze the microbiological data and susceptibility profile.

The pathogen is an ESBL-producing E. coli. While resistant to most beta-lactams, it is susceptible to carbapenems, fluoroquinolones, nitrofurantoin, and fosfomycin.

Identifying susceptible agents is the first step in selecting a step-down therapy.

Apply pharmacokinetic principles to rule out inappropriate susceptible agents.

Nitrofurantoin and fosfomycin are eliminated from consideration because they do not achieve adequate concentrations in the renal parenchyma.

An antibiotic must reach the site of infection; pyelonephritis requires renal tissue penetration, not just bladder concentration.

Apply antimicrobial stewardship principles to finalize selection.

Ciprofloxacin is selected over continuing intravenous ertapenem because it is highly bioavailable orally and effectively targets the susceptible organism.

Transitioning stable patients to appropriate oral therapy minimizes hospital stay length and reduces the risk of central line complications.

Key Concept

In patients with acute pyelonephritis who are clinically stable and ready for step-down therapy, highly bioavailable oral agents like fluoroquinolones are preferred if the isolate is susceptible. Nitrofurantoin and fosfomycin are contraindicated in pyelonephritis regardless of in vitro susceptibility due to inadequate renal tissue penetration.

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