A 71-year-old female resident of a long-term care facility was admitted to the hospital 4 … — USMLE Step 2 CK

A 71-year-old female resident of a long-term care facility was admitted to the hospital $4$ days ago with a temperature of $39.0^{\circ}\text{C}$ ( $102.2^{\circ}\text{F}$ ), rigors, and severe left-sided flank pain. Initial evaluation revealed marked left costovertebral angle tenderness. Laboratory testing demonstrated a leukocyte count of $15,200/\mu\text{L}$ with a left shift. Urinalysis was strongly positive for leukocyte esterase, nitrites, and numerous bacteria. The patient was started on empiric intravenous meropenem and intravenous fluids.

Today (hospital day 4), the patient is hemodynamically stable, has been afebrile for the past $48$ hours, and her flank pain has significantly improved. She is eating a normal diet and is eager to return to her facility. Blood cultures drawn on admission remain negative. The admission urine culture grew $>100,000$ colony-forming units/mL of *Escherichia coli*. Susceptibility testing results for the isolate are as follows:

Antimicrobial Agent	Susceptibility
Ampicillin	Resistant
Ceftriaxone	Resistant
Cefepime	Resistant
Ciprofloxacin	Resistant
Trimethoprim-sulfamethoxazole	Resistant
Nitrofurantoin	Susceptible
Fosfomycin	Susceptible
Ertapenem	Susceptible
Meropenem	Susceptible

Which of the following is the most appropriate next step in pharmacotherapy for this patient?

A
Transition to oral nitrofurantoin
B
Transition to oral fosfomycin
Transition to intravenous ertapenem and arrange for outpatient parenteral therapyCevap
D
Continue intravenous meropenem and arrange for outpatient parenteral therapy
E
Transition to oral ciprofloxacin

Cevap

Transition to intravenous ertapenem and arrange for outpatient parenteral therapy.

The correct decision is to transition the patient to intravenous ertapenem. The patient has pyelonephritis caused by an ESBL-producing E. coli. Because ESBLs hydrolyze most beta-lactams and this isolate is resistant to fluoroquinolones and trimethoprim-sulfamethoxazole, a carbapenem is required. Ertapenem is the drug of choice for step-down therapy because it is the narrowest-spectrum carbapenem (sparing anti-pseudomonal coverage) and its once-daily dosing is ideal for outpatient parenteral antibiotic therapy (OPAT).

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Identify the clinical diagnosis based on the initial presentation.

The patient presented with high fever, rigors, and costovertebral angle tenderness, indicating acute pyelonephritis.

Differentiating between upper (pyelonephritis) and lower (cystitis) urinary tract infections is critical for selecting agents with appropriate tissue penetration.

Analyze the culture and susceptibility results.

The organism is an E. coli isolate resistant to penicillins, cephalosporins (including third- and fourth-generation), and fluoroquinolones, which is classic for an extended-spectrum beta-lactamase (ESBL) producer.

ESBL-producing organisms require specific targeted therapy, typically carbapenems, for invasive infections.

Determine the appropriate step-down therapy for pyelonephritis.

Oral agents like nitrofurantoin and fosfomycin, despite being susceptible in vitro, cannot be used due to poor renal tissue penetration. Intravenous carbapenems remain the standard of care.

Antibiotic selection must account for both organism susceptibility and the pharmacokinetic ability of the drug to reach the site of infection.

Apply principles of antibiotic stewardship to select the best parenteral agent.

Ertapenem is chosen over meropenem because it is a narrower-spectrum carbapenem (it lacks anti-pseudomonal activity) and its once-daily dosing profile is highly favorable for outpatient parenteral antibiotic therapy (OPAT).

De-escalating to the narrowest effective agent prevents the development of broader resistance, and prioritizing once-daily dosing facilitates safe discharge to a long-term care facility.

Anahtar Kavram

While nitrofurantoin and fosfomycin may show in vitro susceptibility against ESBL-producing organisms, they have poor renal tissue penetration and are strictly contraindicated for pyelonephritis. Ertapenem is the preferred step-down carbapenem due to its narrower spectrum and convenient once-daily dosing.

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Cevap

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