LSAT Explanation PT 38, S4, Q22: In humans, ingested protein is broken
LSAT Question Stem
Which one of the following is an assumption on which the argument depends?
Logical Reasoning Question Type
This is a Necessary Assumption question.
Correct Answer
The correct answer to this question is D.
LSAT Question Complete Explanation
First, let's summarize and analyze the argument in the passage. The argument structure is as follows:
Premise 1: Ingested protein is broken down into amino acids, which compete to enter the brain.
Premise 2: Ingestion of sugars leads to insulin production, which clears the bloodstream of amino acids except for tryptophan.
Premise 3: Tryptophan enters the brain and is transformed into serotonin, increasing serotonin levels.
Conclusion: Sugars play a major role in mood elevation, helping one feel relaxed and anxiety-free.
The question type for this problem is Necessary Assumption, which means we need to identify an assumption that the argument depends on for its conclusion to be valid.
An "Evaluate" question for this argument could be: "Does an increase in serotonin levels lead to mood elevation and reduced anxiety?"
Now, let's discuss each answer choice:
a) Elevation of mood and freedom from anxiety require increasing the level of serotonin in the brain.
- This answer choice is incorrect because it states that increasing serotonin levels is necessary for mood elevation and reduced anxiety. The argument assumes that raising serotonin levels is sufficient for these effects, but it does not claim that it is necessary.
b) Failure to consume foods rich in sugars results in anxiety and a lowering of mood.
- This answer choice is incorrect because the argument only discusses what happens when sugars are ingested, not what occurs when they are not ingested.
c) Serotonin can be produced naturally only if tryptophan is present in the bloodstream.
- This answer choice is incorrect because, although the argument states that tryptophan is transformed into serotonin, it does not assume that this is the only way serotonin is produced.
d) Increasing the level of serotonin in the brain promotes relaxation and freedom from anxiety.
- This is the correct answer. The author states that after the ingestion of sugars, more serotonin enters the brain. The author then concludes that this leads to mood elevation and reduced anxiety. Thus, the author assumes that serotonin has an effect on mood and anxiety levels.
e) The consumption of protein-rich foods results in anxiety and a lowering of mood.
- This answer choice is incorrect because the author does not assume this statement. The argument presents a scenario in which the ingestion of protein and the competition of amino acids to enter the brain could result in mood elevation without the ingestion of sugars (e.g., if some of the amino acids entering the brain are tryptophan). Since the author's argument allows for the opposite of this answer choice to occur, it is not an assumption of the argument.