PrepTest 102, Section 2, Question 6
One test to determine whether a person has(86 more words not shown)
Which one of the following(9 more words not shown)
- AAll of the(13 more words not shown)
- BLocalized skin irritations(9 more words not shown)
- CThe ability of(17 more words not shown)
- DSome people who(19 more words not shown)
- EThe body's immune(19 more words not shown)
Reading the argument
Could the test produce the same reaction in someone who hasn't had tuberculosis?
I can follow the proposed mechanism. A previous infection teaches the immune system to recognize certain bacterial proteins. Injecting those proteins later produces the recognition response and a skin irritation. That gives us a reason someone who has been infected would react.
But the physicians go further: everyone who reacts this way must have been infected with tuberculosis.
To make that diagnosis, we need to rule out another route to the same test result. What if a different bacterial infection taught the immune system to recognize the proteins involved? A person could then test positive without the tuberculosis history the physicians infer.
What we’re looking for
Even the initial thought that sensitive skin might react to an injection reminds us to ask about false positives. The precise issue here, though, is this protein-triggered recognition response, not whether skin can ever become irritated for another reason. For the necessary assumption, look for the test's reaction being specific to tuberculosis rather than something another bacterium could also produce.
Working the answers
- A
The test doesn't need the immune system to recognize every protein in every disease-causing bacterium. Recognizing the relevant proteins could be enough to produce the reaction. A general claim about all proteins goes beyond what this test's reasoning requires.
- B
The irritation is a result of the test injection. It needn't be a usual symptom of having tuberculosis. The disease could have other typical symptoms while this test still produces a local response. B confuses a sign caused by the testing procedure with symptoms that ordinarily appear during the illness.
- CCorrect
The proteins' ability to trigger this diagnostic irritation must be specific to the tuberculosis bacterium. If another bacterium could account for the same recognition response, a positive test could reflect that other infection instead. The physicians would no longer be entitled to say everyone reacting this way has been infected with tuberculosis. C addresses the false-positive problem; it doesn't claim that tuberculosis is the only possible cause of any skin irritation.
- D
Contracting tuberculosis because of the injection isn't an assumption needed to use the reaction as evidence of a previous infection. The procedure described injects extracted proteins, not an assertion that it infects the patient. We aren't required to add this harmful effect for the physicians' diagnostic argument to work.
- E
The argument doesn't require a person to have enough bacteria to show obvious disease symptoms before recognition is possible. It relies on the immune system remembering relevant proteins after an infection. Whether overt symptoms are needed concerns another proposed requirement; it doesn't establish that this test response uniquely identifies tuberculosis.
More Necessary Assumption questions
Learn the LSAT our way
Our platform drills Necessary Assumption questions at your level and adjusts as you improve.