Logical reasoning PrepTest 102 · Section 2 · Question 6

Incorrect. The stimulus notes that "certain proteins" are recognized by the immune system, so this answer is too broad in talking about "[a]ll of the proteins."

Incorrect. The stimulus notes that the localized skin irritation is a result of the immune system attacking the TB proteins. This doesn't require a symptom of TB itself to be such irritation.

Correct. Argument or Facts:
Argument

Valid or Flawed:
Flawed

Question Type:
Strengthen with Necessary Premise

Stimulus Summary:
Once someone has a disease, their immune system recognizes and attacks proteins
TB detection - Inject someone with TB proteins and see if there's a reaction

Answer Anticipation:
There's a lot of confusing language in this stimulus, but we shouldn't get lost in the science—instead, we should focus on the logic of it!

We're told to find a necessary premise for the physician's logic, so let's focus there, starting with their conclusion. There, they state that you can test for TB by injecting proteins into a person and looking for a reaction, since someone who has had TB will have an immune system that attacks those proteins.

So, in short, to tell if someone has TB, we inject them with some proteins and see if they get a rash. Where's the assumption there?

Let's think about it this way—let's say someone has TB. They get the injection, and they have a rash. That's a pretty clear indication that they have TB. Now, let's say that someone doesn't have TB. They get the injection. For this method to work, they'd have to not get a rash. But the stimulus doesn't indicate that this is the case. Maybe the immune system learns to attack any weird protein once it gets infected with any bacterium. Maybe there are other bacteria that are related to TB, and a person would react if they had been infected with that other bacteria.

The argument assumes that a reaction to TB proteins indicates TB and not some other disease, so let's find an answer that plays into that assumption.

Answer Explanation:
This answer rules out other bacterium as potentially causing the reaction noted. If other bacteria did cause such a reaction, then this test wouldn't indicate TB, but rather some bacterial infection, undermining the physicians' argument. As such, this answer is a necessary premise and is correct.

Key Takeaway:
When dealing with medical tests, the arguments frequently deal with ideas of false positives and false negatives. Here, the stimulus established that those who had TB will react to the proteins, so false negatives (where a test says someone doesn't have a disease when they do) are more or less ruled out. However, the stimulus doesn't rule out false positives—when someone tests positive but doesn't have the disease. As such, we should have expected an answer here ruling that out—the possibility that someone could test positive despite not having the disease. The correct answer here establishes that by ruling out one type of false positive—that another disease could lead to the same reaction.

Incorrect. First, this answer represents a reason to potentially avoid this test, which cuts against the physicians' argument. Second, even ignoring that, as long as this injection doesn't lead to a false positive or negative, it itself infecting people doesn't change the argument as to whether it could indicate that someone had a preexisting infection.

Incorrect. There's no indication that the symptoms of TB are unique to TB, so there being overt symptoms ahead of the test says nothing as to whether the protein injection method is a reliable test for the disease.