Two weeks after completing a course of oral moxifloxacin for a community-acquired lower re… — USMLE Step 2 CK

Two weeks after completing a course of oral moxifloxacin for a community-acquired lower respiratory tract infection, a $71$ -year-old male presents to the urgent care clinic complaining of profuse, watery diarrhea. He reports having up to $8$ loose bowel movements daily, accompanied by diffuse abdominal cramps and subjective fever. His temperature is $38.2^\circ\text{C}$ ( $100.8^\circ\text{F}$ ), pulse is $102/\text{min}$ , and blood pressure is $114/72\text{ mmHg}$ . Physical examination reveals a mildly distended abdomen with generalized tenderness to palpation, but no guarding or rigidity. Laboratory evaluation shows a leukocyte count of $18,500/\text{mm}^3$ and a serum creatinine level of $1.4\text{ mg/dL}$ (baseline $1.0\text{ mg/dL}$ ). Enzyme immunoassay of the stool is positive for glutamate dehydrogenase antigen and *Clostridioides difficile* toxins A and B. Which of the following is the most appropriate pharmacotherapy for this patient's current condition?

Oral fidaxomicinAnswer
B
Intravenous vancomycin
C
Oral metronidazole
D
Oral loperamide
E
Fecal microbiota transplantation

Answer

The correct intervention is the administration of oral fidaxomicin.

This patient is presenting with an initial episode of severe *Clostridioides difficile* infection (CDI), supported by typical symptoms, recent antibiotic exposure, marked leukocytosis (

> 15,000/\text{mm}^3

), and positive stool assays. Current clinical guidelines strongly recommend the use of oral fidaxomicin or oral vancomycin as the first-line pharmacotherapy for both non-severe and severe initial episodes of CDI. Fidaxomicin is a bactericidal macrocyclic antibiotic with a narrow spectrum of activity specifically targeting *C. difficile*. Its minimal systemic absorption and highly targeted nature help preserve normal enteric flora, significantly reducing the risk of disease recurrence compared to older therapies.

Step-by-Step Solution

Diagnose the patient's condition based on the clinical presentation and laboratory results.

The positive stool assay and recent antibiotic use confirm an initial episode of Clostridioides difficile infection (CDI).

Establishing the specific infectious etiology is necessary to guide targeted antimicrobial therapy.

Stratify the severity of the infection.

The leukocyte count of

18,500/\text{mm}^3

(greater than

15,000/\text{mm}^3

) classifies this as a severe episode.

Historically, severity guided therapy, although current guidelines recommend the same first-line agents for both non-severe and severe initial episodes.

Select the guideline-concordant antimicrobial therapy.

Oral fidaxomicin (or oral vancomycin) is chosen.

Current Infectious Diseases Society of America (IDSA) guidelines recommend narrow-spectrum agents like fidaxomicin over older therapies to maximize cure rates and minimize recurrences.

Key Concept

Management of initial Clostridioides difficile infection

Estimated Time:1m 30s

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