**Core Concept**
Left-sided appendicitis is a condition where the appendix is located in the left lower quadrant of the abdomen, rather than the typical right lower quadrant. This anatomical variation can lead to confusion in diagnosis and treatment. Understanding the embryological development and anatomical variation of the appendix is crucial in recognizing this condition.

**Why the Correct Answer is Right**
The correct answer is that left-sided appendicitis is actually **Meckel's diverticulitis**. This is because Meckel's diverticulum, a remnant of the omphalomesenteric duct, can sometimes be mistaken for appendicitis due to its location in the left lower quadrant. Inflammation of Meckel's diverticulum can mimic appendicitis symptoms. The omphalomesenteric duct is a vestigial structure that usually obliterates during fetal development, but in some cases, it remains patent and forms a Meckel's diverticulum.

**Why Each Wrong Option is Incorrect**
* **Option A:** This is incorrect because left-sided appendicitis is not typically caused by the appendix being located in an abnormal position, but rather by an anatomical variation such as Meckel's diverticulum.
* **Option B:** This is incorrect because sigmoiditis is an inflammation of the sigmoid colon, which is a different condition from appendicitis.
* **Option C:** This is incorrect because diverticulitis of the cecum is an inflammation of the cecum, which is a different condition from appendicitis.

**Clinical Pearl / High-Yield Fact**
It is essential to note that Meckel's diverticulum is a common cause of gastrointestinal bleeding in children, but it can also mimic appendicitis symptoms. A high index of suspicion and a thorough physical examination are necessary to diagnose Meckel's diverticulitis.

**Correct Answer:** D.