Physician: The patient is suffering either from disease X or else from disease Y, but there is no available test for ...
NageMailJuly 30 at 09:11PM
Testing the Assumption
I chose answer C. Keeping in mind that the prompt says because there is 'effective treatment for Y but no treatment for X, we must act on the assumption that the patient has a case of Y," that is the assumption whose truth we are testing.
As there is effective treatment for Y, but not X, by treating the disease as if it were Y, we will know if the disease is Y. This is because if the treatment is ineffective, it must be disease X (as there are no treatments for X.) If the treatment is effective, then we know it is Y (for the opposite reason).
The reasoning against answer C in the description says there is no way to distinguish between the two. There clearly is, however, and it is not only included in the prompt, but it is also the test upon which the assumption depends. The prompt may be at issue here, as it says both that there is no available test for distinguishing X from Y, and that these diseases are distinguishable in virtue of there being effective treatment for one, but not the other (which might be an intended flaw.)
Is this sound reasoning?
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We need a principle that will support the decision to treat based on incomplete information as to the presence of disease X or Y.
The situation here is that we don't know which of two diseases the patient is suffering from but, either way, we only have a treatment for one of the two diseases.
The hidden principle is:
“When two possibilities exist and only one is treatable, assume the treatable one is present and act accordingly.”
Answer choice (C) provides information that is helpful after the tests are administered but this doesn't support making the decision to treat Y when we have no way to confirm it is Y. The stimulus also establishes that there is no way to distinguish before making the decision to treat.
The credited answer (D) instead supports the decision to treat the patient before the decision to treat is made under uncertain conditions.
The flaw in the argument is precisely that it assumes the favorable condition (the patient has Y) without any way to confirm it before acting—hence why the principle in (C) doesn’t strengthen the reasoning, but (D) does.
