**Core Concept**
The question is related to the anatomy and development of the umbilicus (navel or belly button), specifically the process of umbilical separation or retraction. This is a normal physiological process that occurs in infants, and its timing and characteristics are critical in pediatric assessment and diagnosis.

**Why the Correct Answer is Right**
The most common cause of umbilicus not separating at the age of 2 years is **omphalocele**. An omphalocele is a congenital anomaly where the intestines or other organs protrude from the navel due to a defect in the abdominal wall. This condition occurs when the umbilical ring fails to close properly during fetal development. As a result, the umbilicus does not separate, and the protruding organs remain outside the abdominal cavity. The presence of omphalocele is often diagnosed at birth or shortly after, and its management involves surgical repair.

**Why Each Wrong Option is Incorrect**
* **Option A:** Omphalitis is an infection of the umbilicus, which can cause delayed separation but is not the most common cause.
* **Option B:** Umbilical cord abnormalities, like a long or short cord, do not directly affect umbilical separation.
* **Option C:** Gastroschisis is another congenital abdominal wall defect, but it typically presents with a small defect near the umbilicus and does not prevent umbilical separation.

**Clinical Pearl / High-Yield Fact**
A mnemonic to remember the common causes of delayed umbilical separation is: "O" for omphalocele, "I" for intestinal obstruction, and "G" for gastroschisis (though the latter is less common).

**Correct Answer: D. Omphalocele**