**Core Concept:** Gastroileal reflex is a reflex arc involving the gastrointestinal tract that promotes the coordination of colonic contractions following a meal in infants.

**Why the Correct Answer is Right:** The correct answer, **D**, refers to the enteric nervous system (ENS), which is a self-sustained neural network within the gastrointestinal (GI) tract. The ENS regulates the motility and secretion of the GI tract, playing a crucial role in the process of digestion and defecation. In infants, the ingestion of a meal triggers the stimulation of mechanoreceptors within the gastrointestinal tract, initiating the gastroileal reflex. This reflex leads to the coordination of colonic contractions, facilitating defecation following a meal.

**Why Each Wrong Option is Incorrect:**

A. The gastrointestinal tract is part of the autonomic nervous system (ANS), which is separate from the ENS and does not directly relate to the gastroileal reflex.

B. The somatic nervous system (SNS) is responsible for voluntary movements and reflexes involving the skin and muscles, not the gastrointestinal tract or gastroileal reflex.

C. The parasympathetic nervous system (PSNS) is involved in the regulation of the GI tract during digestion and motility, but it is not directly responsible for the gastroileal reflex.

E. The sympathetic nervous system (SNS) is involved in the regulation of cardiovascular and respiratory functions, not the gastrointestinal tract or gastroileal reflex.

**Clinical Pearl:** The gastrointestinal tract's enteric nervous system, specifically the ENS, plays a crucial role in the gastroileal reflex and the coordination of colonic contractions following a meal in infants. This understanding is essential in understanding the mechanisms of digestion, defecation, and bowel habits in infants.