**Question:** Which of the following is not derivative of mesoderm?

A. Skeletal system
B. Circulatory system
C. Endocrine system
D. Nervous system

**Core Concept:** Mesoderm is the second layer of the three germ layers in embryonic development, forming the connective tissues, blood, bone, cartilage, muscle, fat, and blood vessels.

**Why the Correct Answer is Right:** The correct answer, D. Nervous system, is not derived from mesoderm. In the process of neurulation, the neural crest cells arise from the surface ectoderm, which is the third germ layer. Mesoderm contributes to the development of the skeletal system, circulatory system, and endocrine system.

**Why Each Wrong Option is Incorrect:**

A. Skeletal system: Derived from mesoderm, specifically the endoderm layer.
B. Circulatory system: Derived from mesoderm, specifically the mesoderm layer.
C. Endocrine system: Derived from endoderm layer (endoderm gives rise to exocrine glands) and mesoderm layer (endoderm and mesoderm contribute to endocrine cells).

**Clinical Pearl:** Understanding the embryological development of organ systems aids in understanding the pathogenesis of various diseases and conditions, such as those affecting the cardiovascular or endocrine systems, and helps in formulating appropriate treatment strategies.

**Correct Answer:** D. Nervous system