**Core Concept**
Omphalocele is a congenital defect where the intestines or other organs protrude through the navel due to a failure of the abdominal wall to close properly during embryonic development. This defect occurs due to a complex interplay of genetic and environmental factors.

**Why the Correct Answer is Right**
Omphalocele is caused by a failure of the midgut to herniate properly through the umbilical ring during the 6th week of embryonic development. The midgut herniates through the umbilical ring and returns to the abdominal cavity by the 10th week. If the herniation is incomplete or the midgut fails to return to the abdominal cavity, omphalocele occurs. This is often due to a deficiency in the development of the anterior abdominal wall muscles and the failure of the umbilical ring to close properly.

**Why Each Wrong Option is Incorrect**
**Option A:** Omphalocele is not caused by a deficiency in the development of the posterior abdominal wall muscles, which is more characteristic of gastroschisis.
**Option B:** Omphalocele is not caused by a failure of the umbilical cord to form properly, although this can be a contributing factor in some cases.
**Option C:** Omphalocele is not caused by a failure of the midgut to herniate through the umbilical ring, but rather a failure of the midgut to return to the abdominal cavity after herniation.

**Clinical Pearl / High-Yield Fact**
Omphalocele is often associated with other congenital anomalies, such as heart defects, lung hypoplasia, and intestinal atresias, and requires prompt surgical intervention to prevent infection and other complications.

**Correct Answer: D. Failure of midgut to return to the abdominal cavity after herniation.**