**Core Concept**
Meckel's diverticulum is a congenital anomaly resulting from the incomplete closure of the vitelline duct, a structure that connects the yolk sac to the intestinal tract during embryonic development. This remnant is a true diverticulum, containing all layers of the intestinal wall.

**Why the Correct Answer is Right**
Meckel's diverticulum is a vestigial remnant of the vitelline duct, which usually obliterates by the 6th week of gestation. The diverticulum may contain ectopic gastric or pancreatic tissue, leading to complications such as gastrointestinal bleeding, obstruction, or diverticulitis. The presence of ectopic gastric mucosa within the diverticulum can secrete acid, causing ulceration and bleeding.

**Why Each Wrong Option is Incorrect**
**Option A:** This option is incorrect because Meckel's diverticulum is not a remnant of the omphalomesenteric duct, which is a different embryological structure.
**Option B:** This option is incorrect because Meckel's diverticulum is not a remnant of the appendix, although both are intestinal anomalies.
**Option C:** This option is incorrect because Meckel's diverticulum is not a remnant of the umbilical vein, which is a different embryological structure.

**Clinical Pearl / High-Yield Fact**
Meckel's diverticulum is the most common congenital anomaly of the small intestine, occurring in approximately 2% of the population. It is often asymptomatic but can cause complications, particularly if it contains ectopic gastric or pancreatic tissue.

**Correct Answer: D. Vitelline duct.