In humans, ingested protein is broken down into amino acids, all of which must compete to enter the brain. Subsequent...

shunhe on July 20, 2020

Hi @Anna2020,

Thanks for the question! I probably wouldn’t diagram this one, though you could do a pseudo-diagram/summary of parts you found confusing, since the stimulus is somewhat complicated. But there’s not a lot of conditional logic to really diagram.

So let’s summarize what this passage is telling us. We’re told that (excising the important parts) eating sugars makes insulin, which breaks down sugars and gets rid of amino acids except for tryptophan. Tryptophan then enters the brain and turns into serotonin, which increases the serotonin level. The argument then concludes that sugars can play a major role in mood elevation, helping one to feel relaxed and anxiety-free.

Now we’re asked to find an assumption the argument depends on; this is a strengthen with necessary assumption question. And we should see that there’s a gap between serotonin levels and mood elevation, and we should try to bridge the gap with the necessary assumption. Taking a look at (D), that’s what it does. It tells us that increasing serotonin levels promotes relaxation and freedom from anxiety. Let’s test this with the negation test: if this is false, and increasing the level of serotonin doesn’t promote relaxation and freedom from anxiety, then the conclusion doesn’t follow since nothing connects the higher serotonin levels with the better moods, and we only know that the sugars lead to higher serotonin levels. Since negating (D) breaks the argument, we know that it’s a necessary assumption.

Now take a look at (A), which tells us that elevation of mood and freedom from anxiety require increasing the level of serotonin in the brain. Is this a necessary assumption of the argument; does it have to assume (A)? No, it could be true that there are other ways of elevation mood and getting free from anxiety other than increasing the level of serotonin in the brain. That’s perfectly compatible with the argument; the argument assumes that serotonin is sufficient for better moods, but not that it’s necessary for better moods. And so (A) can’t be the necessary assumption and is wrong.

Hope this helps! Feel free to ask any other questions that you might have.