True regarding umbilical cord is :
**Core Concept**
The umbilical cord is a vital fetal structure that connects the fetus to the placenta, facilitating the exchange of oxygen, nutrients, and waste products during pregnancy. It consists of two umbilical arteries and one umbilical vein, which are surrounded by Wharton's jelly, a gelatinous substance that provides cushioning and support.
**Why the Correct Answer is Right**
The correct answer is related to the umbilical cord's function and structure. The umbilical cord is responsible for transporting oxygenated blood from the mother to the fetus and deoxygenated blood from the fetus to the mother. The two umbilical arteries carry deoxygenated blood away from the fetus, while the single umbilical vein carries oxygenated blood from the mother to the fetus. This is a critical aspect of fetal development, as it allows the fetus to receive the necessary nutrients and oxygen for growth and development.
**Why Each Wrong Option is Incorrect**
**Option A:** This option is incorrect because the umbilical cord is not solely composed of muscle tissue. While it does contain smooth muscle, it is primarily composed of connective tissue, including Wharton's jelly.
**Option B:** This option is incorrect because the umbilical cord does not have a significant role in the regulation of fetal breathing. The umbilical cord is primarily responsible for the exchange of gases and nutrients, not the regulation of respiratory function.
**Option C:** This option is incorrect because the umbilical cord is not a significant source of fetal immune cells. While it does contain some immune cells, it is not a primary site for immune function.
**Clinical Pearl / High-Yield Fact**
The umbilical cord is typically 50-60 cm in length at term, and it contains two umbilical arteries and one umbilical vein, which are surrounded by Wharton's jelly. This unique structure allows for the efficient exchange of oxygen, nutrients, and waste products between the fetus and the mother.
**Correct Answer: D.**