The heart of human embryo starts beating

The correct answer is Option 3. In human embryology, the heart is the first functional organ to develop. While the primordium of the heart begins to form earlier through the fusion of endocardial tubes, it officially starts beating and pumping blood during the fourth week of development (approximately 21 to 23 days after fertilization).

• Option 1 & 2: These are incorrect because, during the first three weeks, the embryo undergoes cleavage, implantation, and gastrulation. The vascular system is only just beginning to organize.
• Option 3: This is correct. By the beginning of the fourth week, the heart tube establishes a rhythmic contraction, marking the onset of the circulatory system's functionality.
• Option 4: While a heartbeat is clearly visible via ultrasound by the sixth week, the physiological initiation occurs much earlier, in the fourth week.

For UPSC, it is vital to distinguish between the functional onset (4th week) and clinical detection (usually 6th week onwards).