LSAT 102 | Section 2 | Logical Reasoning: Q6

LR Question 6 Explanation

QUESTION TEXT: One test to determine whether a person has been…

QUESTION TYPE: Necessary Assumption

CONCLUSION: If you react to the injection, you have tuberculosis (TB).

REASONING: Anyone with tuberculosis (TB) reacts to the injection.

ANALYSIS: This sounds like a good argument, but there’s a big flaw. What if some people who don’t have TB also react to the injection? Then the test could only prove who doesn’t have TB.

In medical-speak, there’s a risk the test produces false positives.

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1. It doesn’t matter if the body can recognize all of the proteins in TB. The body only has to recognize the specific proteins used in the test.
2. The argument doesn’t say skin infections are a symptom of TB. It says they’re a symptom of being injected with proteins from the TB bacterium.
3. CORRECT. If other things can cause the skin irritation (i.e. getting a needle jabbed into your arm?), then the test can’t conclusively say if someone has TB. They might have a reaction, but not have TB.
4. Why would we want to test to infect people with TB? That’s horrible. The test is designed to detect TB, not infect people. The argument is better off if this is false.
5. The test will work better if the body can easily recognize bacteria, or proteins from bacteria, in small quantities. We’re presumably injecting smaller quantities.

Recap: The question begins with “One test to determine whether a person has been”. It is a Necessary Assumption question. Learn more about LSAT Necessary questions in our guide to LSAT Logical Reasoning question types.