The earliest sign of foetal life is best detected b:
**Core Concept**
The earliest sign of fetal life is a critical indicator of embryonic development, typically detected by ultrasound or other imaging modalities. The earliest sign is associated with the development of the embryo's cardiovascular system, specifically the **formation of the embryonic heart**.
**Why the Correct Answer is Right**
The correct answer is the earliest detectable sign of fetal life, marking the beginning of embryonic development. This sign is associated with the **formation of the embryonic heart**, which starts to beat around 3-4 weeks post-fertilization. The embryonic heart begins as a cluster of cells that differentiate into the **cardiac primordia**, eventually giving rise to the **truncus arteriosus**, which later divides into the **aorta** and **pulmonary artery**. The presence of cardiac activity is a reliable indicator of fetal viability.
**Why Each Wrong Option is Incorrect**
* **Option A:** This option is incorrect because the earliest sign of fetal life is not related to the development of the gastrointestinal tract, which occurs later in embryonic development.
* **Option B:** This option is incorrect because the earliest sign of fetal life is not associated with the development of the neural tube, which forms the brain and spinal cord.
* **Option C:** This option is incorrect because the earliest sign of fetal life is not related to the development of the limbs, which occur later in embryonic development.
**Clinical Pearl / High-Yield Fact**
The earliest sign of fetal life, detected by cardiac activity, is a critical indicator of embryonic viability. This sign is often used as a benchmark for assessing fetal development and predicting pregnancy outcomes.
**Correct Answer: A. Fetal heart activity.**