In fact,the perfect gentleman drinks medicine that has his dark side___and changes him into the frightening. 答案是exposed 可是對等連接詞前後不是要一樣嘛～??

and 是連接
A) that has his dark side exposed
B) that changes him into ...

"that has his dark side exposed" 是關係子句 用來 形容medicine.它是 <暴露他黑暗面的>藥物 的意思.
"that changes him into the ..." 是關係子句 用來 形容medicine.它是 <把他變成怕的怪物的>藥物 的意思.

that has(連接詞 - and, 前第一個動詞 - 現在式) his dark side exposed (黑暗面被暴露 - 被動性質，故需用過去分詞 - exposed) and changes (連接詞後第二個動詞 - 現在式 - 沒有不一致啊!) him into the frightening
只是不懂是什medicine要用drink?

medicine---S
and------conjunction
that-----you use that in expression such as that of and that which to introduce information or comparisons relating to a noun (medicine) already mentioned.

Medicine
-----(A) is that of changing exposing with his darkside
-----(B) and that of changing him into the frightening (-------)

that---conjunction and relative pronoun uses:-you can use that after nouns to introduce a clause in which you report what someone's words or feelings relate to.
-----(A) that has his dark side exposed
-----(B) that has changed him into a (-------)
Hence relative conjunctions are similiar claimed.