USMLE® Step 1 Question of the Day: Gastrointestinal ischemia

Today’s USMLE® Step 1 question of the day features a 38-year-old woman following an accident involving blood loss, abdominal pain, and bloody stool. What’s the most likely diagnosis? Let’s find out!

A 38-year-old woman is brought to the emergency department following a motor vehicle accident. The patient’s estimated blood loss is around 2 liters. A day later, she began experiencing crampy abdominal pain and multiple bloody bowel movements.

Which of the following regions of the gastrointestinal tract is at the most significant risk for ischemia? 

A. Stomach

B. Duodenum

C. Ileocecal junction

D. Descending colon

E. Rectosigmoid junction

Scroll down for the correct answer!

The correct answer to today’s USMLE® Step 1 Question is…

E. Rectosigmoid junction

Correct: See Main Explanation.

Incorrect Answer Explanations

A. Stomach

Incorrect: The stomach receives extensive blood supply from branches of the celiac artery.

B. Duodenum

Incorrect: The duodenum receives extensive blood supply from branches of the celiac artery and superior mesenteric artery.

C. Ileocecal junction

Incorrect: The ileocecal junction is where the small and large intestines meet. Branches of the superior mesenteric artery supply this region.

D. Descending colon

Incorrect: The descending colon is supplied by branches of the inferior mesenteric artery.

Main Explanation

This patient has symptoms of ischemic colitis following a motor vehicle accident complicated by significant blood loss. Among the answer choices, the rectosigmoid junction is at the most critical risk of developing ischemia.

Ischemia is the suppression or reduction of blood flow to an organ or tissue. The condition can arise due to decreased arterial perfusion (e.g., atherosclerosis, embolic stroke), impaired venous drainage (e.g., Budd-Chiari syndrome, testicular torsion), or shock.

Watershed zones, which are areas of the body that receive dual blood supply from the most distal branches of two arteries, are highly vulnerable to ischemia in the setting of systemic hypoperfusion. By contrast, in cases of arterial occlusion (e.g. embolus obstructing one of the arteries), the watershed regions are spared mainly due to their dual blood supply. Examples of watershed regions include the intestinal splenic flexure and rectosigmoid junction, as well as the boundary areas between the brain’s anterior, middle, and posterior cerebral arteries.

In addition to watershed zones, other areas of the body susceptible to ischemia include the liver’s and renal medulla’s centrilobular zone (since both regions have poor oxygen supply and high metabolic demands) as well as the left ventricular subendocardium (since it can be perfused only during diastole).

Major Takeaway 

Ischemia describes suppressed or reduced blood flow to an organ or tissue. Regions of the body susceptible to ischemia include the watershed zones, centrilobular zone of the liver, renal medulla, and left ventricular subendocardium.

Want to learn more about this topic?

Watch this Osmosis video: Anatomy clinical correlates: Viscera of the gastrointestinal tract

References

• Buckberg, G.D. (1977) Left ventricular subendocardial necrosis. The Annals of Thoracic Surgery. 24(4), 379-393. Doi: 10.1016/s0003-4975(10)63418-2.
• Kalogeris, T., Baines, C.P., Krenz, M., Korthuis, R.J. (2016) Ischemia/reperfusion. Comprehensive Physiology. 7(1), 113-170. Doi: 10.1002/cphy.c160006.
• Ray, S.C., Mason, J., O’Connor, P.M. (2019) Ischemic renal injury: Can renal anatomy and associated vascular congestion explain why the medulla and not the cortex is where the trouble starts?. Seminars in Nephrology. 39(6), 520-529. Doi: 10.1016/j.semnephrol.2019.10.002.

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