Logical Reasoning

PT8 · S4 · Q10 A distemper virus has caused

A free, expert breakdown of this official LSAT Logical Reasoning question.

Save & drill this skill — build targeted practice sets from questions like this one
Video walkthroughs — watch every question solved step by step
81 official LSATs — as questions, timed sections & full-length tests

A distemper virus has caused two-thirds of the seal population in the North Sea to die since May 1988.

Conclusion (clearly / must)

The severe pollution of the North Sea waters must have weakened the immune system of the seals so they could no longer withstand the distemper virus.

Evidence

A distemper virus caused two-thirds of the seal population in the North Sea to die since May 1988.

The explanation for the deaths cannot just be the virus alone. There must be a reason the normally latent virus prevailed so suddenly.

Evaluate

This argument fits the Explain Curious Fact pattern. We have an observed phenomenon: two-thirds of the North Sea seal population dying due to a normally latent distemper virus. Our task is to determine the explanation for this sudden change.

Curious Fact

What caused the normally latent virus to become so deadly to the seals?

Author's Explanation

The severe pollution weakened the seals’ immune systems, allowing the virus to prevail.

The argument assumes there isn't an alternate explanation for why the virus suddenly affected the seals so dramatically. It also assumes the pollution weakening the immune systems is a plausible cause.

Goal

We'll want to seek an answer choice that either rules out an alternate explanation or affirms something needed for the plausibility of the author's explanation involving pollution.

10.

The argument concerning the immune system of the seals presupposes which one of the following?

There has been a gradual
Out of Scope
This answer discusses a historical decline in the seal population over the past two centuries, which isn't connected to the argument about the sudden prevalence of the virus since May 1988. The argument focuses on the recent deaths due to the virus, not long-term population trends.
3% picked this
No further sources of pollution
Opposite (if anything)\
If we negated this, it would sound like there is even more pollution than usual, which would strengthen the author's hypothesis that pollution was a causal factor.
13% picked this
Correct
There was no sudden mutation
Why this is right
When we negate the lovable "no" in this Defender style choice, we get "There was a sudden mutation that allowed the virus to attack healthy seals". Boom, we weakened with an alternate explanation. It was pollution that caused the seals to suddenly be more vulnerable; it was the virus mutating into something much more dangerous."
Skill tested: Necessary Assumption · how this choice captures the argument's function is the move to repeat next time.
77% picked this
Pollution in the North Sea
Irrelevant Comparison
This option compares pollution levels in the North Sea to pollution in other bodies of water. The argument doesn't hinge on such comparisons, as its focus is solely on the impact of pollution in the North Sea on the seal immunity and virus virulence.
1% picked this
Some species that provide food
Too Strong: nearly extinct
Even though the author thinks pollution is bad enough to impact the seals immune system and make them more vulnerable to the virus, she doesn't need to think that other species are on the bring of extinction from the pollution.
6% picked this

Continue the review in LSAT Lab

Save this question, watch the video walkthrough, and drill similar questions in your LSAT Lab account.

Save and practice this skill

LSAT Lab

Turn this review into a targeted study plan.

Save this question, drill more like it, watch the video walkthrough, and track your progress in your LSAT Lab account.

Start practicing free