History 64-year-old woman with 2 days of colicky central abdominal pain, bilious vomiting and distension. Previous open hysterectomy.
Upright abdominal radiograph · Click an image to enlarge.
Answer each question aloud before you reveal it. Marks and domains appear only in the revealed answers, as in the exam.
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Q1 Describe the bowel gas pattern.
Reveal answer
A Annotated 3 marks Observation
- Multiple dilated, gas-filled bowel loops (1)
- Multiple air–fluid levels at differing heights, including within the same loop ('step-ladder' pattern) (1)
- Small bowel features: central distribution, valvulae conniventes where visible (0.5)
- Little gas seen distally in the colon or rectum (0.5)
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Q2 What is your preferred diagnosis? Give two differential diagnoses.
Reveal answer
2 marks Interpretation
- Preferred: mechanical small bowel obstruction (adhesional is most likely after previous surgery) (1)
- Differentials (0.5 each, max 1): paralytic ileus; large bowel obstruction with an incompetent ileocaecal valve; gastroenteritis (1)
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Q3 What is the next investigation, and which features would prompt urgent surgery?
Reveal answer
3 marks Management/Safety
- CT abdomen/pelvis with IV contrast (portal venous phase); positive oral contrast is not needed, as fluid in the bowel acts as contrast (1)
- Find the transition point and the cause (adhesion, hernia, tumour, gallstone) (0.5)
- Signs of ischaemia / strangulation: reduced or absent wall enhancement, wall thickening, mesenteric oedema or fluid, closed-loop configuration, pneumatosis or portal venous gas (1)
- Adhesional SBO without ischaemia: NG decompression and a water-soluble contrast challenge; contrast in the colon within 24 h predicts non-operative resolution (Bologna 2017) (0.5)
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Q4 How do you distinguish small from large bowel on a plain film?
Reveal answer
2 marks Applied sciences
- Small bowel: central; valvulae conniventes cross the full width and are closely spaced; abnormal if > 3 cm (1)
- Large bowel: peripheral; haustra do not cross the full width; contains faeces; abnormal if > 6 cm (caecum > 9 cm): the '3-6-9' rule (1)
End of the OSCER block (10 marks).
Beyond the exam: teaching extension
Q5–Q9: not part of the 10-mark OSCER block. Untimed, viva-style discussion: mimics and pitfalls, complications, guidelines and report wording.
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Q5 Why an upright film? What does the supine film add, and what is the 'string of beads' sign?
Reveal answer
- Air–fluid levels need a horizontal beam (upright or decubitus); the supine film shows gas distribution and loop calibre better
- String of beads: small gas bubbles trapped under the valvulae in fluid-filled loops; strongly suggests SBO
- A near-gasless abdomen can still be SBO when loops are full of fluid; CT is the test
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Q6 What are the causes of SBO?
Reveal answer
- Adhesions (the commonest cause in Australia and similar settings); external and internal hernias
- Tumour, Crohn stricture, intussusception (lead point in adults), volvulus
- Gallstone ileus: Rigler triad of pneumobilia, SBO and an ectopic calcified gallstone
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Q7 What is a closed-loop obstruction, and why does it matter?
Reveal answer
- A loop obstructed at two points (e.g. adhesive band, internal hernia, volvulus)
- CT: C- or U-shaped fluid-filled loop, radial mesenteric vessels converging, whirl sign, two adjacent transition points
- High risk of strangulation, so urgent surgery (Paulson & Thompson 2015)
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Q8 What are the plain-film pitfalls?
Reveal answer
- A few short air–fluid levels can be normal or seen in gastroenteritis
- Early or partial obstruction may still have colonic gas
- Plain-film accuracy depends on reviewer experience (Thompson 2007); CT has replaced it for diagnosis and planning
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Q9 Draft the key sentences of your report.
Reveal answer
- 'Dilated central small bowel loops with multiple air–fluid levels at differing heights and little colonic gas, consistent with small bowel obstruction.'
- 'Recommend CT abdomen/pelvis with IV contrast to find the transition point and assess for ischaemia.'
End of questions. Reveal the diagnosis, key takeaways and syllabus mapping below.
Reveal diagnosis, key takeaways & syllabus mapping
Diagnosis Small bowel obstruction (upright AXR)
Key takeaways
- The plain film suggests SBO; CT confirms it, finds the cause and grades the risk.
- Hunt for ischaemia and closed-loop signs: they send the patient to theatre.
- Know the 3-6-9 rule and the features of small vs large bowel.
- Adhesional SBO without ischaemia: water-soluble contrast challenge (Bologna).
Syllabus mapping
| OSCER station | Abdominal |
|---|---|
| Learning Outcomes section | §6.6 Abdomen & Pelvis |
| Appendix 1 condition | Small bowel obstruction (Appendix 1 F, Small intestine, p73) |
| Category | Category 1 |
| Key Condition | Yes |
| Core marks by domain | Observation 3 · Interpretation 2 · Management/Safety 3 · Applied sciences 2 (= 10) |
| Learning objectives | Recognise SBO on an upright AXR; Choose CT and identify ischaemia and closed-loop signs; Distinguish small from large bowel |
Mapped to the RANZCR Clinical Radiology Learning Outcomes v1.3 (January 2024), section 6 and Appendix 1.
Further reading
- Thompson WM, Kilani RK, Smith BB, et al. Accuracy of abdominal radiography in acute small-bowel obstruction: does reviewer experience matter? AJR Am J Roentgenol. 2007;188(3):W233–W238. PMID 17312028.
- Paulson EK, Thompson WM. Review of small-bowel obstruction: the diagnosis and when to worry. Radiology. 2015;275(2):332–342. PMID 25906301.
- ten Broek RPG, Krielen P, Di Saverio S, et al. Bologna guidelines for diagnosis and management of adhesive small bowel obstruction (ASBO): 2017 update. World J Emerg Surg. 2018;13:24. PMID 29946347.
- RANZCR. Clinical Radiology Learning Outcomes, v1.3 (January 2024), §6 and Appendix 1. Sydney: RANZCR.
- RANZCR. Clinical Radiology Phase 2 OSCER Sample Questions, v1.6 (4 May 2026). Sydney: RANZCR. (Format reference only; no RANZCR images reproduced.)
Always check current Australian/local guidelines.
Image credits
- A by James Heilman, MD, CC BY-SA 3.0, via Wikimedia Commons. Changes: cropped; resized and re-encoded (WebP/JPEG). Teaching arrows and labels are a separate overlay added by this site. Adapted image shared under the same licence.
Original file titles are listed on the image credits page (they may reveal the diagnosis).