**Core Concept**
The process of defecation in infants is influenced by the gastrocolic reflex, a physiological response to food intake that stimulates the colon to contract and move waste towards the rectum. This reflex is mediated by the enteric nervous system and is essential for maintaining regular bowel movements in infants.

**Why the Correct Answer is Right**
The gastrocolic reflex is triggered by the release of certain hormones, such as gastrin and cholecystokinin, which are secreted in response to food entering the stomach. These hormones stimulate the enteric nervous system to release neurotransmitters, including acetylcholine, which acts on the smooth muscle of the colon to cause contraction. The resulting increase in colonic pressure helps to move feces towards the rectum, facilitating defecation. This reflex is an essential mechanism for maintaining regular bowel movements in infants, who have a relatively short and straight colon that relies heavily on peristalsis to move waste.

**Why Each Wrong Option is Incorrect**
**Option A:** This option is not relevant to the gastrocolic reflex or the process of defecation in infants.
**Option B:** This option is incorrect because the gastroileal reflex is a different physiological response that occurs when food enters the small intestine, not the stomach.
**Option C:** This option is incorrect because the vagus nerve does play a role in regulating bowel movements, but it is not the primary cause of colonic contraction in response to food intake.

**Clinical Pearl / High-Yield Fact**
The gastrocolic reflex is an essential mechanism for maintaining regular bowel movements in infants, and its dysregulation can lead to conditions such as constipation or diarrhea.

**Correct Answer: D.**