Clostridioides difficile

1. GC 099 (Antimicrobial Resistance) & GC 106 (Practical Issues in Antibiotic Use): C. difficile is the paradigm organism for antibiotic stewardship — exam questions frequently frame it as the consequence of inappropriate broad-spectrum antibiotic use ^[1]. The lecture emphasis is that clindamycin, fluoroquinolones, and third-generation cephalosporins are the highest-risk antibiotics for triggering CDI.

2. GC 104 (Infection Outbreak / Infection Control): C. difficile is a key healthcare-associated infection requiring contact precautions, soap-and-water hand hygiene, and environmental decontamination with sporicidal agents (bleach) ^[1]. Alcohol gel inefficacy is a favourite MCQ discriminator.

3. GC 102 (Fever After Chemotherapy / Immunocompromised Hosts): In the post-chemotherapy neutropenic patient with diarrhoea, C. difficile must be excluded as a cause — stool toxin assay is mandatory ^[1]. The 2021 SAQ Q12 (see below) directly tests organisms causing diarrhoea post-bone-marrow transplant, where C. difficile is a key answer.

4. CFB WCS27 (Surgical Infection): pseudomembranous colitis and toxic megacolon are surgical emergencies; colectomy may be needed for fulminant C. difficile colitis ^[1].

5. AOS Microbiology: C. difficile toxin detection algorithm (GDH screen → toxin EIA/PCR) is high-yield for diagnostic questions ^[2]. Emphasise that testing should only be done on unformed stool and test-of-cure is not recommended.

6. GC 033 (Chronic Diarrhoea / IBD) and GC 142 (A Child with Loose Stool): C. difficile should be considered in the differential of chronic/relapsing diarrhoea especially in patients with recent antibiotic exposure or hospitalisation ^[1]. In paediatric settings, note that neonates/infants < 12 months can be asymptomatically colonised — testing is generally not recommended in this age group.

• Alcohol hand-rub vs soap-and-water: The single most tested infection control point for C. difficile. Spores are resistant to alcohol. Always choose soap-and-water.
• Metronidazole vs vancomycin: Older guidelines used oral metronidazole as first-line for mild CDI. Current IDSA/SHEA guidelines (and GC lecture framing) prefer oral vancomycin or fidaxomicin as first-line for all severities ^[1][2]. Metronidazole is now reserved as adjunctive IV therapy in fulminant disease.
• NAAT positivity trap: PCR detects the toxin gene, not active toxin. A positive NAAT in a patient with formed stool = colonisation, not infection. This is a common wrong-answer attractor.
• Pseudomembranous colitis on colonoscopy: Yellow-white adherent plaques that cannot be washed off. This is pathognomonic but not present in all CDI cases.
• C. difficile vs C. perfringens: Both are Gram-positive anaerobic spore-forming bacilli. C. perfringens causes gas gangrene (alpha toxin/lecithinase) and a self-limiting food poisoning (enterotoxin). C. difficile causes antibiotic-associated diarrhoea/pseudomembranous colitis. The clinical scenario discriminates them.
• Fecal microbiota transplantation (FMT): Reserved for multiply recurrent CDI (≥ 2–3 recurrences). This is increasingly tested as a "novel therapy" question.

2021 Fourth Summative SAQ Q12 ^[3]:

"A 40-year-old man had bone marrow transplant for chronic myeloid leukaemia. ... (b) On day 28 after neutrophil engraftment, he developed fever, impaired liver function test, persistent diarrhoea and chest X-ray infiltrates. The patient admitted that he did not take the antimicrobial prophylaxis which was prescribed. List [four] organisms which may account for his symptoms."

• Correct answer (for diarrhoea component): Clostridioides difficile is a key answer for the persistent diarrhoea in a post-BMT patient (antibiotic exposure is universal in this setting). Other expected answers include CMV (colitis + hepatitis + pneumonitis), Aspergillus (CXR infiltrates), and Pneumocystis jirovecii (if co-trimoxazole prophylaxis not taken). The question tests recognition that CDI must be excluded in any post-chemotherapy/transplant patient with diarrhoea.

No other past paper questions in the indexed context directly and specifically test C. difficile as the primary organism. However, CDI is a plausible differential in several diarrhoea-related stems (e.g., 2019 Minicase Case 1 — traveller with diarrhoea, though enteric fever is the primary answer there).