A 22-week-old male fetus is evaluated by prenatal ultrasound and found to have a diaphragmatic defect on the left side. The defect allows herniation of abdominal contents into the thoracic cavity. Which of the following embryological structures failed to develop properly to result in this anomaly?

## Embryological Development of the Diaphragm The diaphragm develops from **four main embryological components**: 1. **Septum transversum** — forms the central tendon 2. **Pleuroperitoneal membranes (membranes of Cantrall)** — closes the pleuroperitoneal canals laterally 3. **Dorsal mesentery of the esophagus** — forms the crura 4. **Muscularization of the body wall** — forms the peripheral muscular portions ### Why the Pleuroperitoneal Membrane is Correct The **pleuroperitoneal membranes** are paired structures that grow medially and ventrally to **close the pleuroperitoneal canals** (foramen of Bochdalek) between weeks 8–10 of gestation. - **Failure of closure** results in a **Bochdalek hernia** (posterolateral defect), which is the most common type of congenital diaphragmatic hernia (CDH). - The left-sided location mentioned in the vignette is typical for Bochdalek hernias (left > right, 2:1 ratio). - Herniation of abdominal contents into the thoracic cavity is the direct consequence of this defect. **Key Point:** Bochdalek hernias account for ~90% of all CDH cases and result from failure of the pleuroperitoneal membrane to fuse with the dorsal mesentery of the esophagus. **High-Yield:** The timing of pleuroperitoneal membrane closure (weeks 8–10) is critical; defects occurring after this window are rare.