It is uncertain side effect the medicine will bring about，although about two thousand patients have taken it.
A. that
B. what
C. how
D. whether
网友回答
A. that
(答案→)B. what
C. how
D. whether
解析：B【命题立意】考查主语从句的引导词。
【解题思路】句意为：尽管大约有两千名病人服用过这种药物，但是，它会带来什么样的副作用还不确定。side effect 意为“副作用”，还原主语从句中的主干是the medicine will bring about side effect。故选择what，what在本题中形容词，意为“什么样的”，起修饰作用。