Bunuel
Doctors in the early twentieth century commonly mistook endometriosis as simple menstrual cramps and informed women that there was no medical cure for their condition.
(A) endometriosis as simple menstrual cramps
(B) endometriosis for simple menstrual cramps
(C) simple menstrual cramps for endometriosis
(D) endometriosis to be simple menstrual cramping
(E) endometriosis and simple menstrual cramps
KAPLAN OFFICIAL EXPLANATION:
The sentence describes the misdiagnosis by doctors who thought that one condition (endometriosis) was really another condition (cramps). Since the idiom should be mistook... for, go ahead and eliminate (A), (D), and (E). Choice (C) is wrong because it suggests that doctors thought that cramps were really endometriosis, the opposite of the original meaning. That leaves (B).
An 800 test taker recognizes that many answer choices will be wrong because they distort the meaning of the original sentence. Sometimes necessary grammatical changes will alter the meaning and that's fine, but an unnecessary alteration is always wrong.