A certain medication that is frequently prescribed to lower a patient’s cholesterol level is generally effective. A recent study of 1,000 subjects ranging widely in age indicates, however, that the cholesterol level of someone taking the medication is typically 12 to 15 percent higher than the average for that person’s age group.

"Surprising" Phenomenon
Why would the cholesterol of someone taking the medication be typically higher than the average cholesterol of someone in that person’s age group even though the medication is effective in lowering cholesterol?

Objective
The correct answer should tell us about a difference between people who take the medication and people who don’t that might explain why people who take the medication have higher average cholesterol despite the medication’s effectiveness. For example, maybe people who take the medication start off with above average cholesterol, which could be why the take the medication in the first place. Or maybe people who take the medication engage in cholesterol-increasing activities that the average person does not.

A
A recently developed cholesterol-lowering medication is more effective than the medication described above.
A new, different drug has no impact if we have no reason to think that the average person takes that new drug.
B
Another medication is prescribed to treat high cholesterol just as often as the medication described above is.
A different drug has no impact if we have no reason to think that the average person takes that new drug. Even if we did know that the average person takes a different drug, we don’t know that the different drug is more effective at decreasing cholesterol.
C
In most cases, people with high cholesterol levels are not treated with drug therapy but are put on restrictive low-cholesterol diets.
What happens to most people with high cholesterol doesn’t matter, because we’re trying to explain a discrepancy concerning people who are on the cholesterol-reducing drug.
D
The medication described above is usually prescribed only for people whose cholesterol level is at least 30 percent above the average for their age group.
This tells us people given the medication typically start with cholesterol significantly higher than average. That’s why the drug, even though it helps reduce cholesterol, does not bring the drug-takers’ cholesterol level down to the average person’s cholesterol level.
E
Within the population as a whole, approximately the same number of people have relatively high cholesterol levels as have relatively low cholesterol levels.
The specific number of people with high or low cholesterol does not affect average cholesterol levels of people who take the medication or average cholesterol levels of people who don’t take the medication.

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