Earliest system to be function in fetus is ?
**Core Concept**
The development and maturation of various fetal systems occur at different stages during gestation. The earliest system to be functional in the fetus is crucial for its survival and overall development.
**Why the Correct Answer is Right**
The earliest system to be functional in the fetus is the nervous system, specifically the **neuroectoderm**. This is because the neural tube, which forms the brain and spinal cord, begins to develop and close by the 23rd day of gestation. The neural tube's formation is a critical event in fetal development, and its proper closure is essential for the prevention of neural tube defects. As the neural tube develops, it gives rise to the brain and spinal cord, which eventually mature into functional nervous tissue.
**Why Each Wrong Option is Incorrect**
* **Option A:** The cardiovascular system, although functional early in gestation, is not the earliest system to be functional. While the heart begins to beat around 3 weeks of gestation, it is not the first system to exhibit functional activity.
* **Option B:** The respiratory system is not functional until later in gestation, around 20-24 weeks, when the lungs begin to produce surfactant and are capable of expanding with air.
* **Option C:** The gastrointestinal system, including the liver and pancreas, begins to function later in gestation, around 12-16 weeks.
**Clinical Pearl / High-Yield Fact**
It's essential to remember that the neural tube's closure is a critical event in fetal development, and any disruptions to this process can result in neural tube defects, such as spina bifida or anencephaly.
**Correct Answer: D. Nervous system**