The tendency of the human body to reject foreign matter is the main obstacle to successful organ transplantation.
【正确答案】 D
【答案解析】解析：题干：对外来器官的排斥现象是器官移植的最大障碍。obstacle阻碍，障碍。factor因素，要素；constituent成分，组成要素；break破损，破裂；barrier障碍，阻碍。故答案选D。