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A 67-year-old woman comes to the physician for a health maintenance examination_ Her brother and mother have a history of colon cancer. The physician recommends colonoscopy, but the patient says that she would prefer only for her stool to be tested for blood. The physician explains that testing the stool for occult blood is not appropriate in this case. The physician is most likely concerned about which of the following regarding this test?
A) Low sensitivity
B) Low specificity
C) Potential for a false-positive result
D) Uncertain negative predictive value
E) Uncertain positive predictive value
D is incorrect
Please explain why D ?
if any body can discuss also the other options ?
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another question of NBME 16
To determine whether children of workers in a lead foundry sustain neurological damage from exposure to dust on their parent's clothing, a test for motor skills was given to 30 first graders whose parents has worked at the foundry for 5 years or longer and to 30 controls. Which of the following groups of children would be the proper control group for this study?
A) First graders living with guardians or relatives who worked at the foundry for 5 years or longer.
B)First graders whose parents had never worked at the foundry
C)First graders with motor impairment whose parents had worked at the foundry
D) First graders without motor impairment whose parents had worked at the foundry for 5 years or longer
E) Random sample of school children from the same community
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I think the ans to your first question is A - low sensitivity..because for a good screening test should have a high sensitivity
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Thanks for reply
The answer for 1st q is D , but I don't know why not B
For the 2nd I picked E and it does not show as a wrong answer in the feedback
other people said D or B
I appreciate If anyone can confirm the answer, exam in 3days .
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Is there any generous person can explain the above 2 questions ?
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A for 1st q
B for 2nd q
Explanation 1st q:
Testing stool for blood, the doctor may end up with 4 scenarios:
BLOOD POSITIVE -- CANCER NEGATIVE
BLOOD POSITIVE -- CANCER POSITIVE
BLOOD NEGATIVE - CANCER NEGATIVE
BLOOD NEGATIVE - CANCER POSITIVE
So, in the patient tests BLOOD POSITIVE on occult blood test, the patient should undergo farther investigation.
If negative for cancer - patient goes home happy, if positive for cancer -- surgery, etc...
If OCCULT BLOOD is NEGATIVE and by chance patient is CANCER NEGATIVE - no action needed.
The problem is the last scenario: TEST IS NEGATIVE and patient is CANCER POSITIVE.
Basically you have a FALSE NEGATIVE RESULT ( that excludes choice C)
Occult blood test has a low sensitivity . SNOUT mnemonic. You cannot rule OUT the healthy people.
This is what the physician is worried in this case: he wants to send home a TEST NEGATIVE with CANCER NEGATIVE patient. (choice A), but with only the blood test, he cannot do it.
The test has a low specificity too, but this is the second step (SPIN mnemonic). The physician is not concerned with a POSITIVE BLOOD TEST. At least the patient knows it is positive, and he may undergo further investigation if he wishes to do so.
The NPV and PPV are low, since we have a lot of FP and FN. They used the word " UNCERTAIN" which is not the same as 'low".
Sensitivity and specificity refers to the TEST. NPV and PPV refers to the person (the chances of someone being positive or negative). The question in the above scenario is asking about the TEST , not about the patient . (this way we can rule out choices D and E)
Explanation 2nd q:
They want to find out if -
DUST causes NEUROLOGICAL DAMAGE
The control group should be a group with NO DUST that resembles as much as possible the STUDY GROUP.
Choice B seems the most logical one.
Someone who took it online, please confirm or deny the choices.
Thank you
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I took the test online. A and B are the correct answer choices. Thank you for the explanations.
