A 26-year-old woman is evaluated on the medical ward on hospital day 3. She was initially … — USMLE Step 2 CK

A 26-year-old woman is evaluated on the medical ward on hospital day $3$ . She was initially admitted for severe right-sided flank pain, nausea, vomiting, and a temperature of $39.2^\circ\text{C}$ ( $102.6^\circ\text{F}$ ). On admission, urinalysis revealed marked pyuria and bacteriuria, and she was started on empiric intravenous ceftriaxone for acute pyelonephritis.

Today, she reports significant improvement in her symptoms and is tolerating a regular diet. Her temperature is $37.0^\circ\text{C}$ ( $98.6^\circ\text{F}$ ), blood pressure is $118/76\text{ mmHg}$ , and pulse is $78\text{/min}$ . Physical examination shows resolution of the right costovertebral angle tenderness. Blood cultures drawn on admission show no growth. The urine culture from admission grew $>100,000\text{ CFU/mL}$ of *Escherichia coli*. The isolate susceptibility report is as follows:

Antimicrobial Agent	Interpretation
Ampicillin	Resistant
Cefazolin	Susceptible
Ceftriaxone	Susceptible
Ciprofloxacin	Susceptible
Nitrofurantoin	Susceptible
Trimethoprim-sulfamethoxazole	Susceptible

Which of the following is the most appropriate next step in management?

Prescribe oral trimethoprim-sulfamethoxazole and arrange for dischargeAnswer
B
Prescribe oral ciprofloxacin and arrange for discharge
C
Prescribe oral nitrofurantoin and arrange for discharge
D
Prescribe oral fosfomycin and arrange for discharge
E
Continue intravenous ceftriaxone for an additional 7 days

Answer

Prescribe oral trimethoprim-sulfamethoxazole and arrange for discharge

The correct action is to transition the patient to oral trimethoprim-sulfamethoxazole and arrange for discharge. The patient has met the criteria for IV-to-PO step-down: she is afebrile, hemodynamically stable, symptomatically improved, and tolerating oral intake. Acute pyelonephritis requires an antibiotic that achieves adequate tissue levels in the renal parenchyma. Among the susceptible oral options, TMP-SMX and ciprofloxacin are pharmacokinetically appropriate. However, based on antimicrobial stewardship guidelines, TMP-SMX is the preferred targeted therapy. Fluoroquinolones (ciprofloxacin) are broader-spectrum and carry FDA warnings regarding severe adverse effects (tendinopathy, neuropathy) and increased risk of *C. difficile* infection; they should be reserved for cases where narrower agents are ineffective or contraindicated.

Step-by-Step Solution

Evaluate the patient's clinical trajectory to determine the appropriate setting of care.

The patient is afebrile for over 24 hours, hemodynamically stable, and tolerating an oral diet.

Clinical stability indicates that intravenous antibiotics are no longer necessary and the patient can be stepped down to oral therapy for discharge.

Identify the site of infection to filter pharmacokinetically appropriate antibiotics.

The diagnosis is acute pyelonephritis (upper urinary tract infection).

Antibiotics used for pyelonephritis must achieve high concentrations in the renal parenchyma. Nitrofurantoin and fosfomycin must be eliminated from consideration as they only concentrate in the bladder.

Select the optimal step-down agent based on susceptibility data and antimicrobial stewardship principles.

Between the remaining susceptible options, trimethoprim-sulfamethoxazole is chosen over ciprofloxacin.

Stewardship guidelines mandate reserving broad-spectrum agents like fluoroquinolones for cases where narrower options cannot be used, thereby reducing the risk of resistance, Clostridioides difficile infection, and severe adverse effects (e.g., tendon rupture, QT prolongation).

Key Concept

In patients with acute pyelonephritis who have achieved clinical stability, IV-to-PO step-down therapy should utilize targeted, narrow-spectrum agents with adequate renal tissue penetration (e.g., TMP-SMX) over broad-spectrum agents (e.g., fluoroquinolones) or agents with poor parenchymal penetration (e.g., nitrofurantoin).

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