PrepTest 119, Section 4, Question 17
Researcher: Hard water contains more calcium and magnesium(38 more words not shown)
Which one of the following,(7 more words not shown)
- AMagnesium deficiency is(13 more words not shown)
- BMagnesium is needed(7 more words not shown)
- CAs people age,(6 more words not shown)
- DThe ingestion of(17 more words not shown)
- ECompounds commonly used(15 more words not shown)
Reading the argument
Does the magnesium in the water actually reach the blood?
Hard water contains more magnesium than soft water. The researcher combines that fact with lower blood magnesium among people being treated for certain conditions, then concludes that drinking mostly soft water increases the risk of those conditions. There are several steps packed into that move.
My first question concerns absorption. What if the body couldn't use the magnesium from water, even if it could get magnesium from food? That's a hypothetical possibility, not a fact about nutrition. It shows why different mineral content in water doesn't automatically establish different blood levels or different disease risks.
What we’re looking for
For a weakener, I'd initially look for something breaking that connection. But we should also examine the evidence about the patients. We know they are being treated and have lower magnesium levels; we still need to know what that observation establishes about the cause of their illness.
Working the answers
- A
Magnesium deficiency being common doesn't compare soft-water drinkers with hard-water drinkers. It could be widespread while the proposed difference in risk existed, or while it didn't. We need something bearing on the inference about the water, not just the existence of deficiency.
- B
This gives magnesium a role in preventing increased blood pressure. That fits the researcher's concern about low magnesium rather than undermining it. It doesn't show that the difference between hard and soft water fails to produce the claimed risk.
- C
Older people processing magnesium less effectively introduces an age difference. But we aren't told that the two groups of water drinkers differ in age, or that this explains the patients' low levels. We would have to add a connection before C would address the comparison.
- D
Supplements aren't the same as magnesium in drinking water. And interfering with a medicine's effectiveness isn't the same as increasing the initial risk of developing a condition. D might make us question a blanket claim that more magnesium is always better, but the researcher hasn't made that blanket claim. It doesn't directly explain away the evidence about the patients.
- ECorrect
This takes a different route from my absorption prediction. The treatment itself can reduce the body's ability to absorb and retain magnesium. That gives us another explanation for the low levels observed among people being treated.
The researcher treats low magnesium as a clue to what put them at risk before they became patients. But perhaps they have low magnesium partly because of what happened afterward: they received treatment. Then the observation doesn't establish that low magnesium caused their conditions, much less that drinking soft water increased their risk. E weakens that inference without proving soft water has no effect at all.
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