Rats injected with morphine exhibit decreased activity of the immune system, the bodily system that fights off infections. These same rats exhibited heightened blood levels of corticosteroids, chemicals secreted by the adrenal glands. Since corticosteroids can interfere with immune-system activity, scientists hypothesized that the way morphine reduces immune responses in rats is by stimulating the adrenal glands to secrete additional corticosteroids into the bloodstream.
Which of the following experiments would yield the most useful results for evaluating the scientists’ hypothesis?
(A) Injecting morphine into rats that already have heightened blood levels of corticosteroids and then observing their new blood levels of corticosteroids.
The subjects- rats- already have reduced immunity. This option confirms that corticosteroids affect immunity but it does not confirms or rejects the conclusion that morphine affects immunity.How can we confirm a cause when the effect is already present?
(B) Testing the level of immune-system activity of rats, removing their adrenal glands, and then testing the rats’ immune-system activity levels again.
This option may help confirm that adrenal glands affect immunity by releasing corticosteroids but this option will not help us understand the effect of morphine.
(C) Injecting rats with corticosteroids and then observing how many of the rats contracted infections.
This option may help confirm that corticosteroids affect immunity but this option will not help us understand the effect of morphine.(D) Removing the adrenal glands of rats, injecting the rats with morphine, and then testing the level of the rats’ immune-system responses
Correct as explained below.(E) Injecting rats with a drug that stimulates immune-system activity and then observing the level of corticosteroids in their bloodstreams.
The argument is about effect of morphine and not about any other drug.Cause and effect type CR.
The conclusion Morphine ----> affects adrenal glands-----> release corticosteroids------> reduced immunity.
We are not sure about the above 'chain' of cause and effect because it is possible that Morphine ------directly----> reduces immunity .
Just because the rats in the sample had heightened levels of corticosteroids does not mean that morphine helped induce the heightened levels of corticosteroids.
The best way to confirm the conclusion or reject the conclusion will be remove the adrenal glands and check for immunity levels.
Break the chain and confirm the effect!
Hence, D is correct.
Hope the above is useful.
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