PrepTest 118, Section 4, Question 5
Rats fed high doses of the artificial sweetener(35 more words not shown)
Which one of the following,(14 more words not shown)
- AUrine proteins that(15 more words not shown)
- BCells in the(11 more words not shown)
- CHigh doses of(13 more words not shown)
- DThe silicate crystals(15 more words not shown)
- EHigh doses of(16 more words not shown)
Reading the argument
Before treating a rat study as advice for people, I'd want to know how high the dose was and whether human bodies respond the same way. But this passage doesn't make that leap. It gives us a difference between two animals receiving high doses: rats get bladder cancer, and mice don't.
Follow the steps described for the rats. Saccharin leads to crystals that damage the bladder lining. The cells regenerate, and some of those new cells are cancerous. Where might that sequence break down in mice?
Maybe mice don't develop the crystals. Perhaps their bodies handle the sweetener differently, so the damaging first step never happens. Another possible explanation would be that they die before tumors can develop. Those are possibilities to test, not facts we should assume.
What we’re looking for
The question asks us to explain the different results. I'd look first for a biological difference that stops the crystals from forming in mice. We can accept both observations and explain why the same sweetener doesn't start the same chain in both animals.
Working the answers
- ACorrect
Rats have proteins that react with saccharin to form the crystals; mice lack those proteins. That supplies the biological difference we wanted. The crystal-forming reaction described for rats isn't available in the same way in mice, so we have a reason the mice don't experience the ensuing bladder damage and tumors.
This explains the contrast at the beginning of the chain. We don't need to deny that high doses were given or deny that the rats developed cancer.
- B
Faster regeneration doesn't tell us why cancerous cells fail to appear. In the rats, regeneration is precisely when some cancerous cells emerge. Speeding up that step in mice wouldn't, by itself, establish that the replacement cells are safe. We need a difference that explains the absence of cancer, not simply a faster version of a process involved in it.
- C
Both groups receive high doses. Comparing high doses with lower ones could matter in a different study, but it doesn't distinguish these rats from these mice. The missing explanation concerns their different responses at high doses.
- D
The passage already locates the damage in the bladder lining. Restricting the crystals' toxicity to that location doesn't tell us why one species develops tumors there and the other doesn't. We need a difference between the animals, not between locations within the bladder.
- E
Other sweeteners could have different effects without explaining what saccharin does. This answer reverses the species pattern for other substances, but our question concerns the particular contrast produced by saccharin. We still need a reason for that contrast.
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