**Core Concept**
The underlying condition in this scenario involves a gastrointestinal issue that causes abdominal pain, relieved by defecation in a pediatric patient. This presentation is suggestive of a functional gastrointestinal disorder, specifically a condition characterized by abdominal pain and altered bowel habits.

**Why the Correct Answer is Right**
The diagnosis in this scenario is most likely **Infantile Colic** or **Functional Abdominal Pain**, but given the specific detail that the pain was relieved after passing red stools, it is more likely _**Intussusception**_ or _**Meckel's Diverticulum**_. However, given the age and presentation, **Meckel's Diverticulum** is the most likely diagnosis. Meckel's Diverticulum is a congenital anomaly of the small intestine where a small pouch protrudes from the wall of the ileum, often containing ectopic gastric mucosa that can lead to ulceration and bleeding. The bleeding can present as red stools, which is consistent with the patient's symptoms.

**Why Each Wrong Option is Incorrect**
* **Option A:** Intussusception is a condition where a part of the intestine telescopes into another, leading to bowel obstruction. While it can present with abdominal pain and bloody stools, it typically presents with a palpable abdominal mass and is more common in infants over 3 months old.
* **Option B:** Gastroesophageal reflux disease (GERD) is a condition where stomach acid flows back up into the esophagus, causing symptoms such as regurgitation and abdominal pain. However, GERD is not typically associated with passing red stools.
* **Option C:** Appendicitis is an inflammation of the appendix, which can present with abdominal pain, vomiting, and fever. However, appendicitis is not typically associated with passing red stools.

**Clinical Pearl / High-Yield Fact**
Meckel's Diverticulum is a common congenital anomaly of the small intestine, occurring in approximately 2% of the population. It is often asymptomatic but can present with gastrointestinal bleeding, obstruction, or intussusception.

**Correct Answer:** D. Meckel's Diverticulum.