You’ve probably seen this in a q-bank: a patient with inflammatory diarrhea after undercooked poultry, and the question feels “too easy”… until the answer choices start trying to bait you into every other cause of diarrhea. The real skill isn’t just picking Campylobacter jejuni—it’s knowing why the distractors are wrong fast, using a few high-yield anchors.
Clinical Vignette (Q-Bank Style)
A 22-year-old college student presents with 2 days of fever, crampy abdominal pain, and diarrhea that became bloody. Symptoms started after a weekend cookout where he ate undercooked chicken. Vitals: T 38.6°C (101.5°F). Exam shows diffuse abdominal tenderness. Stool studies reveal fecal leukocytes. Culture grows a curved, motile gram-negative rod that grows best at 42°C in microaerophilic conditions.
Most likely organism?
A. Campylobacter jejuni
B. Vibrio cholerae
C. Shigella dysenteriae
D. Enterotoxigenic E. coli (ETEC)
E. Salmonella enteritidis
F. Yersinia enterocolitica
Correct Answer: Campylobacter jejuni
Why it’s Campylobacter
This stem is basically a greatest-hits album:
- Undercooked poultry exposure (classic)
- Inflammatory diarrhea: fever + abdominal pain + bloody stools + fecal leukocytes
- Curved (“comma/S-shaped”) gram-negative rod
- Motile (polar flagella)
- Microaerophilic (needs reduced oxygen)
- Grows at 42°C (a favorite USMLE “lab clue”)
High-yield microbiology & path
- Virulence: invasion of intestinal mucosa + cytolethal distending toxin (CDT) → cell cycle arrest → mucosal damage → inflammation
- Clinical pattern: can mimic appendicitis due to RLQ pain (ileocecal involvement)
- Common complication: Guillain–Barré syndrome (molecular mimicry of gangliosides)
- Also associated with: reactive arthritis (often in HLA-B27 patients)
Treatment (what USMLE likes)
- Usually self-limited → oral rehydration
- Consider antibiotics for severe/prolonged disease or high-risk patients:
- Azithromycin is typical first-line
- Fluoroquinolone resistance is increasingly common (test writers sometimes hint at travel/resistance patterns)
The Distractors: Why Every Other Choice Is Wrong
Quick “pattern recognition” table
| Organism | Key exposure | Stool type | Classic buzzword(s) | What would need to change in the vignette |
|---|---|---|---|---|
| Campylobacter jejuni | Undercooked poultry, unpasteurized milk | Bloody, fecal WBCs | Curved, 42°C, microaerophilic, GBS | — |
| Vibrio cholerae | Contaminated water, shellfish | Profuse watery (“rice-water”) | ↑cAMP via Gs | No fever/blood; massive volume loss |
| Shigella dysenteriae | Daycare, person-to-person | Bloody + tenesmus | Shiga toxin (inhibits 60S) | More prominent tenesmus; very low inoculum |
| ETEC | Travel, contaminated food/water | Watery | LT (↑cAMP), ST (↑cGMP) | Afebrile traveler’s diarrhea, no fecal WBCs |
| Salmonella enteritidis | Eggs, poultry, reptiles | Watery ± inflammatory | Motile, H2S+ (often) | Often watery; can be inflammatory, but “curved/42°C” points away |
| Yersinia enterocolitica | Pork (chitterlings), unpasteurized milk | Watery or bloody | Appendicitis-like, cold enrichment | Pork + mesenteric adenitis; can mimic Crohn |
B. Vibrio cholerae
Why it’s tempting: “foodborne diarrhea” + gram-negative rod.
Why it’s wrong here:
- Cholera is noninflammatory → typically no fever, no blood, no fecal leukocytes
- Diarrhea is profuse watery (“rice-water”), leading to severe dehydration
High-yield contrast:
- Toxin: cholera toxin ADP-ribosylates Gs → ↑adenylate cyclase → ↑cAMP → ↑Cl⁻ secretion
C. Shigella dysenteriae
Why it’s tempting: bloody diarrhea + fever.
Why it’s wrong here:
- The stem hands you curved rod + 42°C + microaerophilic, which screams Campylobacter
- Shigella is typically described as nonmotile and spread person-to-person (daycare, nursing homes)
- Often causes tenesmus (painful urgency), which isn’t emphasized in the vignette
USMLE toxin hook:
- Shiga toxin inhibits the 60S ribosomal subunit → can cause HUS (more classically with EHEC)
D. Enterotoxigenic E. coli (ETEC)
Why it’s tempting: common cause of diarrhea; appears everywhere.
Why it’s wrong here:
- ETEC causes watery “traveler’s diarrhea,” usually afebrile
- No fecal leukocytes (noninvasive, toxin-mediated)
- Blood is not a typical feature
High-yield toxins:
- LT: ↑cAMP (cholera-like)
- ST: ↑cGMP
E. Salmonella enteritidis
Why it’s tempting: poultry exposure + diarrhea + fever.
Why it’s wrong here:
- Salmonella can cause inflammatory diarrhea, but the culture clue here is decisive:
- Salmonella is not classically described as curved or growing at 42°C in microaerophilic conditions
- Also, Salmonella is often linked to eggs, poultry, and reptile exposure, and may present more as watery diarrhea (though it can become inflammatory)
High-yield add-ons:
- S. typhi: Vi capsule, chronic carriage in gallbladder, rose spots, relative bradycardia
- Nontyphoidal Salmonella: risk of bacteremia in sickle cell, HIV, extremes of age
F. Yersinia enterocolitica
Why it’s tempting: abdominal pain that can mimic appendicitis + bloody diarrhea possible.
Why it’s wrong here:
- Exposure history doesn’t fit: Yersinia screams pork (chitterlings) or unpasteurized milk
- Classic association is mesenteric adenitis/terminal ileitis, sometimes “pseudoappendicitis”
- Lab clue is different: cold enrichment can help isolate Yersinia (another USMLE favorite)
High-yield associations:
- Can cause reactive arthritis
- More common in children; can mimic inflammatory bowel disease
High-Yield Takeaways (What to Memorize)
Campylobacter jejuni “anchors”
- Curved gram-negative rod, motile
- Microaerophilic, grows at 42°C
- Undercooked poultry / unpasteurized milk
- Inflammatory diarrhea: fever, abdominal pain, bloody stools, fecal WBCs
- Complications: Guillain–Barré, reactive arthritis
Inflammatory vs noninflammatory diarrhea (fast sorting)
- Inflammatory (invasion/cytotoxin): fever, blood, fecal leukocytes
- Think: Campylobacter, Shigella, Salmonella, EHEC (blood but classically no fecal leukocytes because toxin-mediated), Entamoeba
- Noninflammatory (enterotoxin): watery, no fever, no fecal leukocytes
- Think: ETEC, cholera, rotavirus, norovirus, Giardia
Rapid-Fire Exam Pearls
- If you see 42°C + curved rod, don’t overthink it: Campylobacter jejuni.
- Poultry + bloody diarrhea + fever is a common trap for Salmonella—use the culture conditions/morphology to break the tie.
- Remember the big complication pairing:
- Campylobacter → Guillain–Barré
- Shiga toxin (Shigella/EHEC) → HUS