By which day do the fetal heart tube fuse and merge?

• A. One
• B. Seven
• C. Fourteen
• D. Twenty One

Answer: (D)

The question tests the timing of heart tube fusion in early development. In the embryo, there are two separate endocardial tubes forming in the cardiogenic region. These tubes migrate toward the midline and fuse to create one primitive heart tube. This fusion happens by the end of the third week after conception (about day 21). Once fused, the single tube will undergo folding and looping to establish the basic heart structure that will become the chambers and connections of the heart.