Each week, Osmosis shares a USMLE® Step 1-style practice question to test your knowledge of medical topics. Today’s case involves a 55-year-old man with alternating episodes of loose stools and constipation over the past several months, with associated cramping abdominal pain. Can you figure it out?
A 55-year-old man presents to his primary care physician due to changes in his bowel habits. The patient reports he has been experiencing alternating episodes of loose stools and constipation over the past several months, with associated cramping abdominal pain. The patient’s past medical history is significant for hypertension, hyperlipidemia, and diabetes. His past surgical history is notable for an appendectomy at age 18. His temperature is 37°C (98.6°F), pulse is 98/min, respirations are 18/min and blood pressure is 137/94 mmHg. BMI is 36 kg/m2. Physical examination shows a distended abdomen without focal tenderness, rebound, or guarding. A fecal occult blood test is positive. The patient undergoes a colonoscopy that demonstrates outpouchings of the colonic mucosa and submucosa in the descending colon. Which of the following best describes the pathophysiology of this disease process?
A. Increased intraluminal pressure at points of vascular penetration
B. Failure of involution of the vitelline duct
C. Telescoping of a proximal colon segment into a distal segment
D. Erosion of adjacent loops of bowel due to full thickness granulomatous inflammation
E. Outpouchings surrounding a vestigial lymphoid structureScroll down to find the answer!
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The correct answer to today’s USMLE® Step 1 Question is…
A. Increased intraluminal pressure at points of vascular penetration
Before we get to the Main Explanation, let’s look at the incorrect answer explanations. Skip to the bottom if you want to see the correct answer right away!
Incorrect answer explanations
The incorrect answers to today’s USMLE® Step 1 Question are…
B. Failure of involution of the vitelline duct
Incorrect: This answer describes the pathophysiology of a Meckel’s diverticulum, which contains all layers of the colonic wall, not just the mucosa and submucosa.Meckel’s diverticulum is a remnant of the omphalomesenteric (vitelline) duct, and it is the most common congenital malformation of the gastrointestinal tract. It usually presents in pediatric patients with painless hematochezia, or it is discovered incidentally during colonoscopy at the terminal ileum.
C. Telescoping of a proximal colon segment into a distal segment
Incorrect: This answer describes the pathophysiology of intussusception, which occurs when a lead point (such as a Meckel’s diverticulum, lymphoid tissue, or tumor) causes the proximal end of bowel to peristalse into the more distal end. Intussusception can present with colicky abdominal pain in both pediatric patients and adults, although it is much more common in the pediatric population. Intussusception is unlikely in this patient without an identifiable lead point visualized on colonoscopy.
D. Erosion of adjacent loops of bowel due to full thickness granulomatous inflammation
Incorrect: This answer describes fistula formation due to Crohn’s disease, which results in full thickness inflammation of all layers of the colon or small bowel. While Crohn’s disease can present with hematochezia or a positive fecal occult blood test, it is generally associated with additional constitutional symptoms such as weight loss, arthralgias and fever.
E. Outpouchings surrounding a vestigial lymphoid structure
Incorrect: Diverticula can form around the appendix resulting in an increased risk of obstruction and subsequent appendicitis; however this patient’s appendix has been removed, and his diverticula are noted in the more distal colon, away from the site of the appendix.
Main Explanation
The patient in this vignette presents with alternating constipation, diarrhea, and colicky abdominal pain with a colonoscopy that reveals outpouchings of the mucosa and submucosa. Together, these findings are consistent with a diagnosis of diverticulosis. Diverticulosis is the presence of multiple false diverticula, or pseudodiverticula, which occur only when the mucosa and submucosa outpouch. They are thought to be due to prolonged increased intraluminal pressure from diets poor in fiber at the weakest points of the gastrointestinal tract, where the vasa recta perforate the muscularis externa. Diverticula may also bleed intermittently, resulting in anemia or a positive fecal occult blood test, or they may become infected, resulting in diverticulitis.
In contrast to false diverticula, true diverticula, such as a Meckel’s diverticulum or inherited diverticula of the appendix, contain all layers of the bowel wall. True diverticula are much less common in comparison to false or pseudodiverticula.
Major Takeaway
Diverticulosis is an example of a false diverticulum that only contains the mucosa and submucosa. It is thought to be due to prolonged increased intraluminal pressure at the weakest points of the gastrointestinal tract due to low dietary fiber intake.
References
Curry, M. (2017, May 18). Rosen’s Emergency Medicine: Concepts and Clinical Practice. Retrieved from https://www.us.elsevierhealth.com/rosens-emergency-medicine-concepts-and-clinical-practice-9780323354790.html.
Everhart JE, Ruhl CE. Burden of digestive diseases in the United States part II: lower gastrointestinal diseases. Gastroenterology. 2009;136(3):741‐754. doi:10.1053/j.gastro.2009.01.015
Sagar J, Kumar V, Shah DK. Meckel’s diverticulum: a systematic review [published correction appears in J R Soc Med. 2007 Feb;100(2):69]. J R Soc Med. 2006;99(10):501‐505. doi:10.1258/jrsm.99.10.501
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