PrepTest 152, Section 1, Question 22
Although severing a motor nerve kills part of(91 more words not shown)
The statements above, if true,(8 more words not shown)
- ADoubling the speed(22 more words not shown)
- BIt is sometimes(17 more words not shown)
- CIf a severed(24 more words not shown)
- DIf living nerve(27 more words not shown)
- EWithout surgical intervention,(18 more words not shown)
Reading the argument
The distance matters. A nerve growing from the shoulder toward a hand muscle has much farther to travel than one growing from the wrist toward that same muscle. At about a millimeter a day, that difference could add a lot of time.
Then we learn about a second problem: the growing cells need the original sheath as a guide, and the sheath starts disintegrating after about three months unless living nerve tissue is inside it. The nerve needs time to grow, but the structure guiding it may not remain intact.
Roughly three months of growth would cover about 90 millimeters. That helps us picture the timing, but it isn't a strict rule that every longer repair must fail. Disintegration begins at that point; we aren't told how quickly it finishes. There is also the stated exception for a sheath containing living tissue.
What we’re looking for
For the most-supported answer, keep those limits in view. We can connect the need for a guide with the threat to that guide without inventing an exact deadline for every recovery.
Working the answers
- A
Twice the growth speed doesn't establish twice the likelihood of success. Suppose a particular short gap could already be crossed well before the sheath became a problem. Faster growth might reduce the time without doubling the chance of reaching the muscle. The passage gives a growth rate, not a mathematical rule relating changes in speed to changes in probability.
- B
The exception concerns living tissue preventing the sheath from beginning to disintegrate. It doesn't establish that we can reverse or slow disintegration once it has begun. That might be possible, but the passage doesn't tell us. We shouldn't turn preventing a process into an established ability to undo it.
- C
Three months is when disintegration begins, not when every sheath necessarily vanishes or all regeneration becomes impossible. The living-tissue exception also matters. And the passage doesn't rule out every other way functioning might be restored. C turns a threat to this process into an absolute deadline for any restoration of function.
- DCorrect
This uses the living-tissue exception. If implanted tissue could be kept alive within the sheath, the sheath would have protection against the disintegration described. Since growing nerve cells require that sheath as a guide, preserving it could improve their chance of reaching the muscle in cases where losing the guide would otherwise interfere.
We don't need to know that such an implantation is currently possible or that it would guarantee recovery. The answer is conditional, and it promises only an increased likelihood in some cases. That is appropriately limited support from the two facts about the sheath: it is needed, and living tissue can keep it from disintegrating.
- E
We're told a severed nerve can regenerate; we aren't told surgery is generally necessary for that process to restore function. A short distance or a preserved sheath might allow regeneration without the intervention E assumes. The passage doesn't establish that recovery without surgery is unlikely.
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