53.1.3 Science columnist: It is clear

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I think I am making this one way harder than it needs to be, but I have been spinning my wheels for a half hour on this one. I don't understand how B weakens the argument? The conclusion only states that "it is clear why humans have some diseases in common with cats." So what if B is true? What about the some diseases that humans have in common with cats that do have a genetic basis? B to me is completely consistent with the argument. The argument isn't concluding that ALL diseases or MOST of the diseases are common. I have watched the video on this one 2-3 times, and I am still dumbfounded how B even slightly weakens the argument.

c.janson35

At its core, the argument is saying, "Humans and cats have so many diseases in common because cats are so close genetically to humans, and many human diseases are genetically based!" So, the author is concluding that the diseases share a genetic kinship in both humans and cats, but he or she is taking this relationship completely for granted. What if all of the cats' diseases are not genetically based? This is still a possibility from what we know in the argument because the argument does not provide us with any information that the diseases that are talked about overlap between the two groups at all, even if the genomes do. Moreover, "many human diseases are genetically based" could very well mean "one human disease is genetically based", and in this case, are we still confident in saying that the reason why cats and humans have so many diseases in common is because cats are genetically similar to humans, and one human disease is genetically based? No, not at all.

Essentially, the author is assuming there is a large overlap between the genetic diseases that humans and cats have, but this not need be the case. Answer choice B exposes this assumption. If most (which could also mean all) diseases that the two groups have in common have no genetic bases, then how could genetics be used to describe why the groups have so many diseases in common?

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@c.janson35 Thanks!

Is this breakdown accurate do you think? It's pretty rough, but I think it gets the idea across.

"Many human diseases are genetically based" translates to Human Disease SOME Human gene.

"Each of the genes ID'd in cats are in humans" translates to Cat Gene----->Human gene

Cat Gene------->Human Gene SOME Human Disease? So we don't have any inferences with this, but the author is saying Cat Disease SOME Human Disease? If B is true, then this even further weakens the already weak existential modifier chain?