PT132 · S4 · Q19 Doctor: Medical researchers recently

This is tempting. If it said "A factor that sometimes doesn't present a certain effect to arise may nonetheless be sufficient to produce that effect in other cases", it would match. Our evidence is about people who had the Cause (bad disks) but not the Effect. Saying "a factor need not be present in order for an effect to arise" is referring to a case in which you had the Effect, but not the Cause. If this argument had been, "These people who have back pain don't have bad disks. Thus bad disks can't cause back pain", then this answer would have matched.

In this conversation, Cause = Bad disks, and Effect = Back Pain. So this reads like this: Back pain that occurs in the absence of bad disks might not occur when there are bad disks. That doesn't match the evidence. We had bad disks with an absence of back pain. And the Anti-Conclusion we're trying to argue is that an effect (back pain) might occur for other people. This answer's Anti-Conclusion is that back pain might not occur.

What was the characteristic found in half of the sample? They had bulging / slipped disks. Was the author assuming or leaning on the idea that half of the entire population has bulging / slipped disks? No, she didn't care what % of the population has bad disks. She's concluding that bad disks don't cause back pain, regardless of the percentage of the population that has bad disks.

The first part matches the evidence: we had a factor (bad disks) that did not bring about a certain effect (back pain). But this answer is saying that "bad disks may still be more likely present than not when back pain occurs". The issue of the conclusion is not whether bad disks are associated with back pain, but whether they cause back pain. Saying that "bulging / slipped disks are more likely to be found in someone with back pain" does not establish that the bad disks caused the back pain.