This is a real-world medical entrance exam question, please give the true answer based on the question and selection. Topic name: Introduction and gametogenesis., Subject name: Anatomy
Which of the following is the origin of the mitochondrial DNA of all human adult cells? 
Select and explain. 
A. Paternal only, B. Maternal only, C. A combination of paternal and maternal, D. Either paternal or maternal
B: Maternal only
Answer: B: Maternal only 
Explanation: The mitochondrial DNA of all human adult cells is of maternal origin only. In human feilization, the entire sperm enters the secondary oocyte cytoplasm. However, sperm mitochondria degenerate along with the sperm's tail. Therefore, only mitochondria present within the secondary oocyte (maternal) remain in the feilized zygote.