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#21 - When a person with temporal lobe epilepsy is having an

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Complete Question Explanation

Must Be True. The correct answer choice is (D)

The data says: If a person with TLE has an epileptic seizure, the brain's TL produces abnormal electrical impulses which can often be detected by EEG.

There are two possible problems with the EEG test. One is the "false positive": the EEG might detect abnormal electrical impulses when there's not actually any abnormal impulses, or there are abnormal impulses but there is no seizure.

Another problem with the EEG test is the "false negative": the EEG might fail to detect abnormal electrical impulses when they are actually present, caused by a seizure.

The last sentence of the stimulus suggests that a positive EEG reading is a reasonably reliable indicator of TLE … and what goes in the blank? A natural concept to go in the blank is: there's a small chance that it might just be a false positive.

Answer choice (A): If a positive EEG reading indicates 50% probability of TLE and 50% probability of no TLE, then this is not a "reasonably reliable" test, as the stimulus said. So this answer choice cannot be correct.

Answer choice (B): The stimulus never discussed other forms of epilepsy, so this answer choice cannot be correct.

Answer choice (C): We know nothing about the frequency of false positives vs. false negatives.

Answer choice (D): This is the correct answer choice. This answer choice captures the "false negative" phenomenon as discussed above.

Answer choice (E): If both a positive EEG reading and a negative EEG reading give exactly the same information about the likelihood of TLE, then running the test is completely pointless. We don't have any information from the stimulus to support this.
acp25
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Hi,

I read the explanation and I just don't get it. I understood the stimulus up until "Therefore..." Which part of the stimulus did the two possible EEG problems come from?
Jay Donnell
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Hi acp25!

This is a nasty MBT fill in the blank style question, and I'm happy to help clear it up!

The key behind the whole question comes from the following statement: "...produces abnormal electrical impulses, which can often, but not always, be detected through a test..."

This means that the EEG test can't always catch the incidence of temporal epilepsy, so if a test failed to register as a positive result, it doesn't mean that the patient doesn't suffer from temporal epilepsy.

The concept illustrated here involves what is often referred to as a Confusion Matrix, which entails the four possible results of a test for a given condition.

Let's say that we are developing a test that identifies whether or not a patient has the gene for mutant super powers. Okay, you got me, I am not-so-secretly very excited for Avengers Endgame to come out next week!

So, if we ran this test on a patient we have only four possible outcomes:

True Positive: The test found the patient to have the mutant gene, and the patient actually does have the gene.
True Negative: The test found the patient to not have the mutant gene, and the patient actually doesn't have the gene.

False Positive: The test found the patient to have the mutant gene, but the patient actually does not have the gene.
False Negative: The test found that the patient did not have the mutant gene, but the patient actually does have the gene.


The risk in this particular stimulus involves a False Negative. Since we know that the EEG test can "often, but not always detect" temporal lobe epilepsy, it's possible that a test that shows up as negative could be mistaken, and the patient does in fact have the condition.

We know that if the test results come out as positive it IS a trustworthy indicator of the presence of the condition (True Positive), but that there remains a potential risk factor of a False Negative diagnosis.

Just because the test was negative it doesn't mean the patient does not have the condition, it simply could have been one of the instances in which the test was simply unable to detect the presence of temporal lobe epilepsy in the patient.


I hope that helps to clear it up, happy Friday to you! :-D 8-)