PT152 S1 Q22 ExplanationAlthough severing a motor nerve

Answer choices, explained

1. A

   Too Strong: doubling = doubling4% picked this

   Doubling the speed at which new nerve cells grow will double the likelihood that a severed motor nerve will reach

   Doubling the speed will certainly increase the likelihood that the motor nerve will reach the muscle it had controlled, but we can't derive such an exactly proportional idea as "double the growth speed, double the probability". (Keep in mind that something with a likelihood of, say, 60% cannot be doubled, or else you'd be saying, nonsensically, that it had a 120% chance of happening).

   4%
2. B

   Out of Scope: reverse/slow disintegration7% picked this

   It is sometimes possible, once a nerve sheath has begun to disintegrate, to reverse or slow

   We only hear about the sheath disintegrating in the last sentence, and we only hear that it will begin to disintegrate after about three months. There's no information that suggests that once that occurs, it can be reversed or slowed.

   7%
3. C

   Too Strong: cannot be restored21% picked this

   If a severed motor nerve does not regenerate successfully within three months after being severed, functioning cannot be restored to the muscle

   After 3 months, we know that the sheath begins to disintegrate. Can we say that once it begins to disintegrate, functioning cannot be restored? It's a little too harsh. There might still be enough sheath to finish off the re-growing process. Or maybe the muscle never lost functioning from the one severed motor nerve.

   21%
4. D

   Correct67% picked this

   If living nerve tissue could be implanted and sustained within the original sheath of a severed motor nerve, the likelihood that the nerve will

   Why this is right

   This is very weakly worded: in some cases, if we could do this, the likelihood would increase (in at least one situation, the likelihood goes up at least somewhat). Since we know that ~living tissue → disint. sheath → ~guide we know that not-having living tissue could be a reason that nerves would fail to grow back. Since the re-growth timeline is so slow (1mm / day) and injuries could easily be more than 90 mm from the injury site (i.e. a shoulder is definitely more than 9cm from a hand), there are certainly some cases in which the nerve doesn't have time to grow back before the sheath would disintegrate. So if we could get living tissue in there so that the sheath had a longer timeline before disappearing, we could probably give the re-growing motor nerve a better chance of making it all the way back.

   Skill tested: Most Supported · how this choice captures the argument's function is the move to repeat next time.

   67%
5. E

   Out of Scope1% picked this

   Without surgical intervention, a muscle that has lost function because of a severed motor nerve is unlikely

   Out of Scope: surgical intervention Too Strong: unlikely We never talked about "surgical intervention", and we don't have good reason to be so pessimistic that we think the muscle will usually not (unlikely to) regain function.

   1%