A 72-year-old man with CKD stage 3 and on a proton pump inhibitor develops severe pseudomembranous colitis due to *Clostridioides difficile* infection after 10 days of ceftriaxone and levofloxacin. Laboratory studies show WBC 24,000/uL, creatinine 2.8 mg/dL (baseline 1.4), and flexible sigmoidoscopy confirms characteristic yellow-white pseudomembranes. The treatment regimen marked **A** in the diagram is most appropriate for this patient. Which of the following best describes why this regimen is preferred over the alternatives in severe CDI?
A. It can be given intravenously in fulminant disease and provides better systemic coverage than oral agents
B. It is rapidly absorbed systemically and achieves high serum levels that penetrate the colonic mucosa to eliminate vegetative bacteria
C. It achieves high intraluminal colonic concentration and directly inhibits C. difficile toxin production, making it effective for severe disease per IDSA/SHEA 2021 guidelines
D. It has superior activity against gram-negative anaerobes and reduces secondary bacterial overgrowth in the colon
Explanation
Why option 1 is correct
The structure marked A — oral vancomycin 125 mg QID or fidaxomicin 200 mg BID — is the IDSA/SHEA 2021 first-line treatment for both non-severe and severe CDI. These agents work by achieving high intraluminal colonic concentrations that directly inhibit C. difficile and suppress toxin production. Vancomycin is a glycopeptide antibiotic that binds to bacterial cell wall precursors, while fidaxomicin is a macrocyclic antibiotic with superior C. difficile coverage and lower recurrence rates. In this patient with severe CDI (WBC ≥15,000 and creatinine ≥1.5 mg/dL), oral vancomycin or fidaxomicin is the evidence-based standard of care.
Why each distractor is wrong
Option 2: While vancomycin is poorly absorbed orally, this is actually an ADVANTAGE—it remains in the colonic lumen where C. difficile resides. The question asks why the regimen is preferred, not why it has poor absorption. Systemic absorption is not the mechanism of benefit here.
Option 3: C. difficile is a gram-positive anaerobe; the regimen targets C. difficile directly, not gram-negative organisms. Secondary bacterial overgrowth is not the pathophysiologic basis for choosing vancomycin/fidaxomicin over metronidazole in severe disease.
Option 4: Intravenous vancomycin alone is explicitly INEFFECTIVE in CDI because it does not reach the colonic lumen in therapeutic concentrations. The regimen marked A is oral, not intravenous. This option confuses a contraindicated approach (IV vancomycin monotherapy) with the correct one.
High-YieldNEET PG
Oral vancomycin/fidaxomicin achieve high intraluminal colonic drug levels; IV vancomycin does NOT reach the colon and is ineffective for CDI.
IDSA/SHEA Clinical Practice Guidelines for C. difficile Infection 2021 Update
Practice similar questions
Sign up free to access AI-powered MCQ practice with detailed explanations and adaptive learning.
