A 71-year-old woman is evaluated in the hospital for new-onset diarrhea. She was admitted … — USMLE Step 2 CK

A $71$ -year-old woman is evaluated in the hospital for new-onset diarrhea. She was admitted $8$ days ago for a severe chronic obstructive pulmonary disease exacerbation, for which she was treated with intravenous methylprednisolone, nebulized bronchodilators, and a $5$ -day course of oral levofloxacin. Over the past $48$ hours, she has developed crampy abdominal pain and $6$ to $8$ watery bowel movements daily. Her temperature is $38.1^\circ\text{C}$ ( $100.6^\circ\text{F}$ ), blood pressure is $112/74\text{ mmHg}$ , and pulse is $98/\text{min}$ . Physical examination shows mild, diffuse abdominal tenderness without guarding or rebound. Laboratory studies reveal a leukocyte count of $18,500/\text{mm}^3$ and a serum creatinine of $1.4\text{ mg/dL}$ (baseline $0.9\text{ mg/dL}$ ). Stool enzyme immunoassay is positive for *Clostridioides difficile* glutamate dehydrogenase antigen and toxins A and B. Which of the following is the most appropriate targeted pharmacotherapy for this patient?

Oral fidaxomicinAnswer
B
Intravenous vancomycin
C
Oral metronidazole
D
Intravenous metronidazole
E
Oral rifaximin

Answer

The most appropriate targeted pharmacotherapy is oral fidaxomicin.

The correct answer identifies the preferred first-line agent for an initial episode of Clostridioides difficile infection (CDI). Current guidelines strongly recommend oral fidaxomicin due to its narrow spectrum, minimal systemic absorption, and significantly lower rates of disease recurrence compared to alternative historical agents.

Step-by-Step Solution

Analyze the clinical presentation and diagnostic laboratory findings.

The patient developed new-onset fever, abdominal pain, and diarrhea after a recent course of antibiotics, and tested positive for C. difficile toxins and antigen.

Establishes the diagnosis of a primary Clostridioides difficile infection (CDI).

Determine the severity of the infection to guide pharmacotherapy.

The leukocyte count of

18,500/\text{mm}^3

and acute rise in serum creatinine indicate severe CDI, but she lacks hemodynamic instability, toxic megacolon, or ileus, ruling out fulminant CDI.

Severity classification dictates the appropriate agent, route, and intensity of antibiotic therapy.

Select the appropriate pharmacotherapy based on the severity and current clinical guidelines.

Guidelines recommend oral fidaxomicin (or oral vancomycin) as the first-line therapy for an initial episode of non-fulminant CDI.

Targeted luminal therapy is strictly required; systemic antibiotics (like IV vancomycin) or historically used agents (like metronidazole) are either pharmacologically ineffective or clinically inferior.

Key Concept

The first-line treatment for an initial episode of Clostridioides difficile infection is oral fidaxomicin (or oral vancomycin). Intravenous vancomycin is ineffective because it does not cross the colonic mucosa into the bowel lumen.

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