**Core Concept**
Ehler Danlos syndrome is a group of genetic disorders characterized by hypermobile joints, skin hyperextensibility, and tissue fragility. This condition is primarily caused by defects in the production or structure of collagen, a key protein that provides strength and elasticity to connective tissue.

**Why the Correct Answer is Right**
The correct answer is related to the genetic mutations that affect the production or structure of collagen. Specifically, Ehler Danlos syndrome is often caused by mutations in the COL5A1, COL5A2, and COL3A1 genes, which encode for the alpha chains of type V collagen. These mutations lead to the production of abnormal collagen molecules that are either defective or absent, resulting in the characteristic features of the syndrome.

**Why Each Wrong Option is Incorrect**
**Option A:** This option is incorrect because Ehler Danlos syndrome is not primarily caused by defects in elastin, a protein that provides elasticity to connective tissue, but rather by defects in collagen.
**Option B:** This option is incorrect because while osteogenesis imperfecta is also a genetic disorder affecting connective tissue, it is caused by defects in type I collagen, not the same type of collagen affected in Ehler Danlos syndrome.
**Option C:** This option is incorrect because Marfan syndrome is a separate genetic disorder caused by defects in the FBN1 gene, which encodes for fibrillin-1, a protein involved in the formation of elastic fibers.

**Clinical Pearl / High-Yield Fact**
It's essential to remember that Ehler Danlos syndrome is a heterogeneous disorder with multiple subtypes, each with distinct clinical features and genetic causes. A thorough understanding of the genetic basis of the syndrome is crucial for accurate diagnosis and management.

**Correct Answer: D. Defects in type V collagen genes (COL5A1, COL5A2, and COL3A1) are the primary cause of Ehler Danlos syndrome.**