## **Core Concept**
The superior mesenteric artery (SMA) is a vital blood vessel that supplies oxygenated blood to the midgut, which includes the distal half of the duodenum, jejunum, ileum, cecum, ascending colon, and the proximal two-thirds of the transverse colon. Occlusion of the SMA can lead to ischemia and infarction of these areas.

## **Why the Correct Answer is Right**
The jejunum and ileum, which are parts of the small intestine, receive their blood supply from the superior mesenteric artery. When the SMA is occluded, it affects the blood supply to these parts of the intestine, leading to ischemia. The jejunum is particularly susceptible to ischemic injury due to its high metabolic rate and demand for oxygen and nutrients. Therefore, occlusion of the SMA would indeed affect the jejunum.

## **Why Each Wrong Option is Incorrect**
- **Option A:** This option is incorrect because the areas primarily affected by SMA occlusion include the jejunum and ileum, not just the stomach and proximal duodenum, which are supplied by the celiac trunk and the gastroduodenal artery, respectively.
- **Option B:** This option is incorrect because while the SMA occlusion does affect a significant portion of the intestines, stating it affects the "whole small intestine" is inaccurate. The SMA supplies the midgut (distal half of the duodenum to two-thirds of the transverse colon), not the entire small intestine (which includes the foregut and midgut).
- **Option C:** This option is incorrect because it inaccurately describes the effect of SMA occlusion. The SMA primarily affects the midgut derivatives, which include the jejunum and ileum but not the entire gastrointestinal tract uniformly.

## **Clinical Pearl / High-Yield Fact**
A key clinical pearl to remember is that acute mesenteric ischemia due to SMA occlusion presents with severe abdominal pain that is out of proportion to the physical examination findings, often described as "pain out of proportion." This condition requires prompt diagnosis and intervention to prevent bowel infarction.

## **Correct Answer:** C. jejunum and ileum.