Critical Reasoning

Which of the following most logically completes the passage?

Appendicitis (inflammation of the appendix) is potentially fatal; consequently, patients with symptoms strongly suggesting appendicitis almost have their appendix removed. The appropriate surgery is low-risk but performed unnecessarily in about 20 percent of all cases. A newly developed internal scan for appendicitis is highly accurate, producing two misdiagnoses for every 98 correct diagnoses. Clearly, using this test, doctors can largely avoid unnecessary removals of the appendix without, however, performing any fewer necessary ones than before, since _______.

A. the patients who are correctly diagnosed with this test as not having appendicitis invariably have medical conditions that are much less serious than appendicitis
B. the misdiagnoses produced by this test are always instances of attributing appendicitis to someone who does not, in fact, have it
C. all of the patients who are diagnosed with this test as having appendicitis do, in fact,
have appendicitis
D. every patient who is diagnosed with this test as having appendicitis has more than one of the symptoms generally associated with appendicitis
E. the only patients who are misdiagnosed using this test are patients who lack one or more of the symptoms that are generally associated with appendicitis

GMATGuruNY wrote:

Which of the following most logically completes the passage?

Appendicitis (inflammation of the appendix) is potentially fatal; consequently, patients with symptoms strongly suggesting appendicitis almost have their appendix removed. The appropriate surgery is low-risk but performed unnecessarily in about 20 percent of all cases. A newly developed internal scan for appendicitis is highly accurate, producing two misdiagnoses for every 98 correct diagnoses. Clearly, using this test, doctors can largely avoid unnecessary removals of the appendix without, however, performing any fewer necessary ones than before, since _______.

A. the patients who are correctly diagnosed with this test as not having appendicitis invariably have medical conditions that are much less serious than appendicitis
B. the misdiagnoses produced by this test are always instances of attributing appendicitis to someone who does not, in fact, have it
C. all of the patients who are diagnosed with this test as having appendicitis do, in fact,
have appendicitis
D. every patient who is diagnosed with this test as having appendicitis has more than one of the symptoms generally associated with appendicitis
E. the only patients who are misdiagnosed using this test are patients who lack one or more of the symptoms that are generally associated with appendicitis

Premise: The new scan produces two misdiagnoses for every 98 correct diagnoses.
Conclusion: If doctors use the new scan, they will not be performing any fewer necessary surgeries than before.

Conclusion, rephrased:
If doctors use the new scan, they will still be performing the same number of NECESSARY surgeries.

For the conclusion to be valid, what must be true?
It must be true that the new scan will NOT weed out any patients for whom surgery is NECESSARY.
B: The misdiagnoses produced by this test are always instances of attributing appendicitis to someone who does not, in fact, have it.
Since the misdiagnoses are not attributed to patients who are actually ill, the new scan will not weed out patients for whom surgery is necessary, strengthening the conclusion that doctors will still be performing the same number of necessary surgeries.

The correct answer is B.

Mo2men wrote:Dear Mitch,

Does the OA in this question considered a 'strengthener' or 'must be true'? In your answer above you mentioned both.