**Core Concept:**
Meckel's diverticulum is a vestigial structure present in the human body, formed by incomplete closure of the umbilical artery during embryonic development. It is a saccular outpouching of the mucosa in the ileum, most commonly found in the antimesenteric border of the ileum. The Rule of 2 is a principle describing the types of appendages found in vertebrates:

1. Only one type of appendage is present in each vertebrate class (e.g., fish have fins, birds have wings, mammals have limbs).
2. Each appendage type is present in an odd number of vertebrate classes.

**Why the Correct Answer is Right:**
Meckel's diverticulum follows the Rule of 2, as it represents an odd-numbered structure present in humans (vertebrate class). In contrast, the given options do not follow the Rule of 2:

A. This option is incorrect as it does not follow the Rule of 2, as humans have only one type of appendage (limbs) and not an odd number.
B. This option is incorrect as it does not follow the Rule of 2, as humans have only one type of appendage (limbs) and not an odd number.
C. This option is incorrect as it does not follow the Rule of 2, as humans have only one type of appendage (limbs) and not an odd number.
D. This option is correct because it follows the Rule of 2, as humans have one type of appendage (limbs) and an odd number (humans have 2 appendages).

**Why Each Wrong Option is Incorrect:**
A. This option is incorrect because it does not follow the Rule of 2, as humans have only one type of appendage (limbs) and not an odd number.
B. This option is incorrect because it does not follow the Rule of 2, as humans have only one type of appendage (limbs) and not an odd number.
C. This option is incorrect because it does not follow the Rule of 2, as humans have only one type of appendage (limbs) and not an odd number.

**Clinical Pearl:**
The Rule of 2 is an important principle in vertebrate anatomy that helps to remember the types of appendages (limbs, fins, tails, etc.) in various vertebrate classes. This principle is useful for understanding the development and homology of appendages across different vertebrate classes.