Which of the following structures is derived from the midgut and therefore lies on the mesentery after normal rotation?

Explanation

## Embryologic Origin and Peritoneal Attachment of Gut Structures ### Midgut Derivatives **Key Point:** The midgut gives rise to all structures that remain **mobile on a mesentery** after normal rotation: jejunum, ileum, cecum, appendix, and ascending colon (proximal 2/3 of colon). ### Classification of Gut Derivatives by Embryologic Origin | **Embryologic Origin** | **Structures** | **Peritoneal Attachment** | |---|---|---| | **Foregut** | Pharynx to proximal duodenum, stomach, liver, pancreas (dorsal lobe) | Retroperitoneal (duodenum) or suspended on mesentery | | **Midgut** | Distal duodenum to proximal 2/3 colon (jejunum, ileum, cecum, appendix, ascending colon) | **Mobile on mesentery** | | **Hindgut** | Distal 1/3 colon, rectum, upper anal canal | Retroperitoneal or suspended on mesentery | ### Why Midgut Structures Remain on Mesentery 1. The midgut herniates into the umbilical cord during weeks 6–10. 2. During rotation around the SMA, the midgut structures acquire a **long mesentery** (the mesentery of the small intestine and mesocolon). 3. After rotation, these structures retain their **mobile peritoneal attachment**, unlike the duodenum and colon (which become retroperitoneal). **High-Yield:** A quick way to identify midgut derivatives: "**J-I-C-A-A-C**" = **J**ejunum, **I**leum, **C**ecum, **A**ppendix, **A**scending colon, and the distal part of the **C**olon (proximal 2/3). All are mobile on mesentery. **Mnemonic:** **"JICAAC on a Mesentery"** — these are the midgut structures that remain suspended and mobile after rotation. **Clinical Pearl:** The mobile mesentery of midgut structures predisposes them to **volvulus** (twisting), a common cause of bowel obstruction in children. Retroperitoneal structures (duodenum, ascending/descending colon) are less prone to volvulus.