This is a real-world medical entrance exam question, please give the true answer based on the question and selection. Topic name: CVS Embryology, Subject name: Anatomy
Which of the three primary germ layers forms the histologically definitive endocardium of the adult hea? 
Select and explain. 
A. Ectoderm, B. Endoderm, C. Mesoderm, D. Epiblast
C: Mesoderm
Answer: C: Mesoderm 
Explanation: C. The entire cardiovascular system is of mesodermal origin.