**Core Concept**
Meckel's diverticulum is a congenital anomaly of the small intestine, resulting from the incomplete closure of the vitelline duct during embryonic development. This leads to a small, pouch-like structure, usually containing gastric or pancreatic mucosa, which can cause various symptoms depending on its location and size.

**Why the Correct Answer is Right**
The most common presentation of Meckel's diverticulum is gastrointestinal bleeding, often due to the presence of ectopic gastric mucosa within the diverticulum. This gastric mucosa can secrete acid, leading to ulceration and subsequent bleeding. The bleeding is typically painless, intermittent, and may be massive. The blood loss can be significant, and in some cases, it may be the first manifestation of the condition. The ectopic gastric mucosa is responsible for producing pepsin and hydrochloric acid, which can cause inflammation and bleeding in the adjacent intestinal tissue.

**Why Each Wrong Option is Incorrect**
**Option B:** Obstruction is a possible complication of Meckel's diverticulum, but it is not the most common presentation. Obstruction may occur if the diverticulum becomes inflamed or if there is a twist in the intestine, but these situations are less common than bleeding.
**Option C:** Diverticulitis is inflammation of the diverticulum, which can occur due to bacterial infection. However, diverticulitis is less common than bleeding as a presentation of Meckel's diverticulum.
**Option D:** Intussusception is a condition where one part of the intestine telescopes into another, which can be caused by a variety of factors, including Meckel's diverticulum. However, intussusception is not the most common presentation of Meckel's diverticulum.

**Clinical Pearl / High-Yield Fact**
It's essential to remember that Meckel's diverticulum can cause bleeding due to the presence of ectopic gastric mucosa. This can be a cause of obscure gastrointestinal bleeding, and the presence of Meckel's diverticulum should be considered in the differential diagnosis of upper or lower gastrointestinal bleeding.

✓ Correct Answer: A. Bleeding