A 28-year-old pregnant woman at 16 weeks gestation undergoes routine ultrasound. The fetus shows absence of the dorsal mesentery of the duodenum. Which of the following embryological structures failed to develop normally?

## Embryological Basis of Duodenal Mesentery ### Development of the Duodenum The duodenum is derived from both **foregut** (proximal 1/3) and **midgut** (distal 2/3). The mesenteries of the duodenum have distinct embryological origins: - **Proximal duodenum (D1–D2):** Retains ventral mesentery (hepatoduodenal ligament) - **Distal duodenum (D3–D4):** Loses its dorsal mesentery during rotation of the midgut ### The Dorsal Mesentery of the Duodenum The **dorsal mesentery of the duodenum** is derived from the **dorsal pancreatic bud** and its associated peritoneal fold. During normal embryological development: 1. The dorsal pancreatic bud arises from the duodenum around week 4 2. It carries with it a fold of peritoneum (dorsal mesentery) 3. This mesentery normally fuses with the posterior peritoneum during weeks 6–8, obliterating the dorsal mesentery **Absence of the dorsal mesentery** indicates failure of the dorsal pancreatic bud to develop normally, as the mesentery is an integral part of the pancreatic bud's peritoneal attachment. ### Key Point: **The dorsal pancreatic bud and its mesentery are inseparable embryologically.** Failure of the dorsal pancreatic bud results in absence of the dorsal duodenal mesentery. ### Clinical Pearl: This embryological defect may be associated with: - Pancreatic agenesis (if both buds fail) - Annular pancreas (if ventral bud rotates abnormally) - Duodenal obstruction (if mesenteric anomalies persist)