Yersinia enterocolitica
Gram-negative, facultatively anaerobic coccobacillus of the family Yersiniaceae that causes enterocolitis, mesenteric lymphadenitis, and pseudoappendicitis, particularly in children.
Organism Card
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| Classic traps |
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Exam Intelligence
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Penetrating enteric infection pattern: GC / Paeds lecture slides classify Y. enterocolitica under "penetrating" infections of the distal small bowel, alongside typhoidal Salmonella and Campylobacter fetus — expect fecal mononuclear leukocytes, not PMNs [2].
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Reactive arthritis trigger: Multiple senior notes and the Felix Lai Medicine notes [4] list Yersinia as one of the five classic enteric pathogens causing reactive arthritis (Reiter's): Salmonella, Shigella, Yersinia, Campylobacter, C. difficile — associated with HLA-B27, seronegative spondyloarthropathy.
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Erythema nodosum: Ryan Ho Rheumatology notes [5] explicitly list Yersinia alongside Strep, TB, Salmonella, and Campylobacter as infectious causes of EN — bilateral tender shin nodules with prodromal fever and arthralgia.
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Cold enrichment / 4 °C growth: This is the single most-tested lab feature. It explains both the CIN agar culture technique and the transfusion sepsis scenario (stored RBC bags at 4 °C).
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Iron-overload synergy: Patients with haemochromatosis or those on desferrioxamine chelation are uniquely susceptible because desferrioxamine acts as a xenosiderophore for Yersinia, dramatically increasing virulence.
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Pseudoappendicitis in exams: A young patient (adolescent/young adult) with RLQ pain, fever, and preceding diarrhoea who goes to surgery and has a normal appendix but inflamed mesenteric lymph nodes → Y. enterocolitica mesenteric adenitis is the expected answer.
- Yersinia vs Appendicitis: Pseudoappendicitis from Y. enterocolitica typically has antecedent diarrhoea and mesenteric lymphadenopathy on imaging; true appendicitis has periumbilical-to-RLQ migration pain without prominent diarrhoea.
- Yersinia vs Crohn's: Terminal ileitis on imaging may look identical; clinical history of acute onset + culture positivity differentiates infectious ileitis from IBD.
- Y. enterocolitica vs Y. pestis: Both are Yersinia but vastly different. Y. pestis = plague (buboes, pneumonic plague, flea-bite transmission, non-motile, urease −); Y. enterocolitica = food-/water-borne enteritis (motile at 25 °C, urease +). The 2022 MCQ EMQ [6] lists Yersinia pestis as a distractor among opportunistic organisms — do not confuse the two species.
- Transfusion sepsis differential: Organisms that contaminate stored blood products include Y. enterocolitica (RBC units, 4 °C) and Staphylococcus epidermidis / Serratia (platelet units, room temperature). Vignette of post-transfusion rigors + shock with G−ve rod in blood → Y. enterocolitica.
- Reactive arthritis pathogen list: When asked for "enteric organisms causing reactive arthritis," give the classic five — do not list E. coli or Vibrio (common wrong answers).
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2022 Fourth Summative MCQ Q19–21 (EMQ) [6]: The option list includes "Yersinia pestis" as a distractor alongside other organisms in an immunocompromised-patient EMQ. While the correct answers for Q19–21 are Talaromyces marneffei, Streptococcus mitis, and CMV respectively, Y. pestis was placed as a distractor. This tests whether students can differentiate Y. pestis (plague, not opportunistic in HIV) from other choices. Y. enterocolitica was not listed but the inclusion of Yersinia in the option bank highlights genus-level awareness expected.
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No other directly indexed past-paper questions specifically test Y. enterocolitica by name in the provided past-paper context. However, the organism is highly examinable via reactive arthritis triggers (Felix Lai Medicine notes [4]) and erythema nodosum causes (Ryan Ho Rheumatology [5]), both of which are testable as SAQ list-type questions.
[1] Lecture slides: GC 105. Medically important microbes what every doctor should know.pdf; GC 142. A child with loose stool.pdf [2] Senior notes: MBBS Final MB (Pediatrics) (Felix PY Lai).pdf — p.328 (enteric infection classification table) [3] Lecture slides: GC 195. Lower and diffuse abdominal pain RLQ problems; pelvic inflammatory disease; peritonitis and abdominal emergencies.pdf [4] Senior notes: MBBS Final MB (Medicine) (Felix PY Lai).pdf — p.1715 (Reactive arthritis etiology) [5] Senior notes: Ryan Ho Rheumatology.pdf — p.157 (Erythema nodosum causes) [6] Past papers: 2022 Fourth Summative MCQ.pdf — p.40 (EMQ Q19–21 option list including Yersinia pestis)
Shigella
Gram-negative, non-motile, facultatively anaerobic bacillus of the family Enterobacteriaceae that causes bacillary dysentery (shigellosis) characterized by invasive bloody, mucoid diarrhea.
Pseudomonas aeruginosa
Obligate aerobic, oxidase-positive, non-fermenting Gram-negative bacillus that is a leading cause of nosocomial infections, particularly ventilator-associated pneumonia, burn wound infections, and opportunistic infections in immunocompromised and cystic fibrosis patients.