key ‧ stone /ˈkiːstəʊn $ -stoʊn/ BrE AmE noun
1 . [countable] the large central stone in an ↑ arch that keeps the other stones in position
2 . [singular] formal the most important part of an idea, belief, or process that influences how it develops
keystone of
The keystone of any personal injury case is medical evidence.