**Question:** Not mesodermal in origin is

A. Endoderm
B. Ectoderm
C. Mesoderm
D. Epigenome

**Core Concept:** The three primary germ layers in early embryonic development are ectoderm, mesoderm, and endoderm. These layers give rise to various tissues and organs in the body.

**Why the Correct Answer is Right:** Option A (Endoderm) is mesodermal in origin, as it is derived from the mesoderm layer during embryonic development.

**Why Each Wrong Option is Incorrect:**

Option B (Ectoderm) is also mesodermal in origin, contrary to the question's premise. In fact, ectoderm is derived from the ectoderm layer.

Option C (Mesoderm) is actually the correct answer, as it is the germ layer responsible for the formation of various tissues and organs, including those derived from endoderm and ectoderm.

Option D (Epigenome) refers to the inheritable changes in gene expression caused by modifications to DNA and histones, rather than being a germ layer in embryonic development.

**Clinical Pearl / High-Yield Fact:** Understanding the germ layers' origin is essential in understanding human development, embryology, and various diseases. For example, diseases like neural tube defects (such as spina bifida) result from improper development of the ectoderm layer.

**Correct Answer:** B. Ectoderm