LSAT Explanation PT 25, S2, Q6: One test to determine whether a

LSAT Question Stem

Which one of the following is an assumption on which the physicians' reasoning depends? 

Logical Reasoning Question Type

This is a Necessary Assumption question. 

Correct Answer

The correct answer to this question is C. 

LSAT Question Complete Explanation

Let's first analyze the argument in the passage. The passage discusses a test designed to determine if a person has been infected with tuberculosis. The test involves injecting the person with proteins extracted from the tuberculosis bacterium. The immune system of a person who has been infected with a bacterium will recognize certain proteins in that bacterium and attack it, resulting in skin irritation at the injection site. The physicians who designed the test concluded that anyone who reacts with skin irritation to the injection of tuberculosis proteins must have been infected with tuberculosis.

The structure of the argument is as follows:

Premise: Once a person has been infected by a bacterium, the person's immune system subsequently recognizes certain proteins present in that bacterium and attacks the bacterium.

Premise: This recognition also takes place in the test and results in a skin irritation at the injection site.

Conclusion: Anyone who reacts with skin irritation to an injection with the tuberculosis proteins has been infected with tuberculosis.

Now let's come up with an "Evaluate" question about the argument: "Is the skin irritation caused exclusively by the tuberculosis bacterium?"

The question type of this problem is Necessary Assumption, which asks us to identify an assumption on which the physicians' reasoning depends.

a) This answer choice is incorrect because the argument does not require that all proteins present in disease-causing bacteria be recognized by the body's immune system. The argument only concerns the recognition of certain proteins in the tuberculosis bacterium.

b) This answer choice is incorrect because the argument is not concerned with whether localized skin irritations are a symptom of tuberculosis in most people. The argument focuses on the skin irritation caused by the test, not by the disease itself.

c) This is the correct answer. The physicians' reasoning depends on the assumption that the ability of the proteins present in the tuberculosis bacterium to trigger the skin irritation is exclusive to that bacterium. This means that no other bacterium shares the same proteins that cause the skin irritation. If this assumption were not true, the test could produce false positives, indicating that a person has tuberculosis when they actually have a different bacterium with similar proteins.

d) This answer choice is incorrect because the argument does not depend on the assumption that some people will contract tuberculosis as a result of the injection. The focus of the argument is on whether the test can accurately determine if someone has been infected with tuberculosis, not on whether the test could cause the disease.

e) This answer choice is incorrect because the argument does not depend on the assumption that the body's immune system cannot recognize infectious bacteria unless there are sufficient quantities of the bacteria to cause overt symptoms of disease. The argument is focused on the recognition of certain proteins in the tuberculosis bacterium, not on the quantity of bacteria needed to cause symptoms.

In summary, the correct answer is (c), as the physicians' reasoning depends on the assumption that the skin irritation caused by the test is exclusive to the tuberculosis bacterium. This ensures that the test accurately determines if a person has been infected with tuberculosis and does not produce false positives due to other bacteria with similar proteins.