Hi All,
As recently as 1950, tuberculosis was never curable
unless sequestered in sanitariums; today, the drug Isoniazid has made such treatment obsolete.

The meaning of the word “sequester” is to “seclude”. So the meaning that is suggested by the sentence is that TB was not curable unless the disease was sequestered. This is illogical. Hence, we need an entity in the sentence that can be sequestered.
POE:Choice A: unless sequestered:
Incorrect for the reason stated above.
Choice B: without sequestering:
Incorrect. This option fails to mention without sequestering whom.
Choice C: without being sequestered:
Incorrect. Same error as in A.
Choice D: unless it was sequestered:
Incorrect. Pronoun “it” refers to “tuberculosis”, making the sentence illogical.
Choice E: unless patients were sequestered:
Correct answer. Hope this helps.
Thanks.
Shraddha