not one of them is/are?

not one of them is/are?
one of them is...这我知道
但我好像又看到not one of them用的是are...
not one of them 到底用is还是are呢?
哦，貌似问错了
找到原题了
Not one of the doctors who ( ) treating my sister knows exactly what is wrong with her.
由此可知 not one至少是knows。
那么这个who的定语从句到底跟的的是doctors还是not one呢？
如果把题目改成
One of the...
又应该如何呢？