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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?
| Text from Passage | Analysis |
| Rats injected with morphine exhibit decreased activity of the immune system, the bodily system that fights off infections. |
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| These same rats exhibited heightened blood levels of corticosteroids, chemicals secreted by the adrenal glands. |
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| 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. |
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The passage starts with an observed effect (morphine weakens immunity), adds a correlated observation (morphine increases stress hormones), then proposes a mechanism connecting these two findings through a chain of causation.
Scientists believe morphine reduces immune responses by stimulating adrenal glands to release more corticosteroids, which then interfere with immune system activity.
This follows a hypothesis-formation pattern: Observation 1 (morphine → weak immunity) + Observation 2 (morphine → high corticosteroids) + Background knowledge (corticosteroids → weak immunity) = Proposed mechanism (morphine → adrenal stimulation → corticosteroids → weak immunity)
Evaluate - We need to find an experiment that would test whether the scientists' hypothesis is correct or not. The hypothesis is that morphine reduces immune responses by stimulating adrenal glands to secrete more corticosteroids.
The hypothesis creates a specific causal chain: morphine → adrenal glands → more corticosteroids → weakened immune system. We need to test this precise mechanism, not just whether morphine affects immunity (we already know that).
To evaluate this hypothesis, we need experiments that can isolate and test the proposed causal chain. We should think of ways to either confirm or disprove that adrenal glands and corticosteroids are the actual pathway through which morphine weakens immunity. The best experiments would either block part of this pathway or test it directly.
This experiment injects morphine into rats that already have high corticosteroid levels and observes corticosteroid changes. However, this doesn't test whether adrenal glands are the pathway through which morphine affects immunity. It only examines corticosteroid levels, not the immune system response that the hypothesis is trying to explain.
This experiment removes adrenal glands and tests immune activity before and after removal, but doesn't involve morphine at all. While it might show that adrenal glands affect immunity, it doesn't test the specific hypothesis about how morphine works through this pathway.
This experiment injects corticosteroids directly and observes infections. This might confirm that corticosteroids can cause infections, but it doesn't test whether morphine actually works through the adrenal-corticosteroid pathway. We already know corticosteroids can interfere with immunity.
This is the most direct test of the hypothesis. By removing adrenal glands first, we eliminate the proposed pathway (\(\mathrm{morphine} \rightarrow \mathrm{adrenal\ glands} \rightarrow \mathrm{corticosteroids}\)). Then injecting morphine and testing immune response tells us whether adrenal glands are truly necessary for morphine's immune-suppressing effect. If immunity still decreases without adrenal glands, the hypothesis is wrong. If immunity remains normal, the hypothesis is supported.
This experiment uses an immune-stimulating drug and observes corticosteroid levels. This tests a reverse relationship and doesn't directly evaluate whether morphine works through the proposed adrenal-corticosteroid mechanism to suppress immunity.