**Core Concept**
Allanto-enteric diverticulum, also known as Meckel's diverticulum, is a congenital anomaly that arises from the failure of the omphalomesenteric duct to close during embryonic development. This duct connects the yolk sac to the midgut and usually obliterates by the 6th week of gestation.

**Why the Correct Answer is Right**
The allanto-enteric diverticulum is formed by the outgrowth of the mucosa and submucosa from the ileum, which is the distal part of the small intestine. This outgrowth occurs due to the incomplete closure of the omphalomesenteric duct, resulting in a pouch-like structure that protrudes from the antimesenteric border of the ileum. The diverticulum may contain ectopic gastric or pancreatic tissue, which can lead to complications such as gastrointestinal bleeding or diverticulitis.

**Why Each Wrong Option is Incorrect**
**Option A:** This option is incorrect because the allanto-enteric diverticulum is not formed by the outgrowth of the muscularis layer of the intestine.
**Option B:** This option is incorrect because the allanto-enteric diverticulum is not formed by the outgrowth of the mesentery of the intestine.
**Option C:** This option is incorrect because the allanto-enteric diverticulum is not formed by the outgrowth of the appendix.

**Clinical Pearl / High-Yield Fact**
It is essential to remember that Meckel's diverticulum is the most common congenital anomaly of the small intestine and can cause complications such as gastrointestinal bleeding, diverticulitis, or obstruction.

**Correct Answer: C.**