A 66-year-old woman presents with abdominal pain and distension. Imaging shows distended bowel loops with haustral markings. Which etiology is most likely the cause of this large bowel obstruction?

• A. Volvulus
• B. Adenocarcinoma
• C. Diverticular disease
• D. Strangulated hernia

Answer: (B)

Adenocarcinoma is the most likely cause because this presentation points to a large-bowel obstruction in an older adult, and the colon is the site of obstruction suggested by the presence of haustral markings on imaging. In the colon, distension proximal to a narrowing is a classic pattern when a malignant lesion gradually encroaches on the lumen. Colorectal cancer, especially adenocarcinoma, is a common cause of large-bowel obstruction in this age group.

Volvulus can cause large-bowel obstruction but typically shows a twisted loop pattern on imaging (often a coffee-bean sign) rather than just distended loops with preserved haustrations. Diverticular disease is another possible cause, but while it can lead to obstruction, cancer remains the more frequent and primary cause of obstruction in an older patient. Strangulated hernia usually presents with signs of hernia and often involves small-bowel obstruction rather than a colon-level process.

So the benchtop reasoning is: older patient, large-bowel obstruction pattern with colonic haustra, and the most common obstructing lesion in this setting is an adenocarcinoma narrowing the colon lumen.