Logical ReasoningDifficulty: Hard

PT113 S4 Q13 Explanation

When several of a dermatologist’s

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

TopicsStrengthen

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Full official LSAT questions are available through LawHub. This page provides LSAT Lab's explanation, strategy, and review tools without republishing the full official question.

Stimulus

When several of a dermatologist’s patients complained of a rash on just one side of their faces, the dermatologist suspected that the cause was some kind of external contact. In each case it turned out that the rash occurred on the side of the face to which the rash was caused by prolonged contact with telephones.

What this question is testing

Strengthen

Conclusion

The dermatologist's call: the rash was caused by holding telephones against the face.

Evidence

The rash showed up on whichever side of the face the patient held the phone to.

Evaluate

This is a Strengthen EXCEPT — four answers will help the diagnosis, one won't. Common ways to strengthen a causal claim: (1) give a plausible mechanism (e.g., the phone material is allergenic), (2) rule out other causes (other facial contact happened on both sides), (3) show dose-response (more phone use = more rash), or (4) show the timing fits (rash appeared after the patient started using phones more).

The odd one out will be a fact that's true about telephones generally but doesn't actually link these patients' rashes to their telephone use. A fact that's equally compatible with telephones causing rashes and telephones not causing rashes doesn't add support.

Goal

Find the answer that doesn't actually shift the evidence one way or the other on whether telephones caused the rashes.

Reading along? Open the full official question in LawHub — we show a fragment here and keep the reasoning in our own words.

The question
13.

Each of the following, if true, provides additional support for the dermatologist’s

Answer choices

  1. Strengthens2% picked this

    Many telephones are now manufactured using a kind of plastic to which some people

    This provides a plausible mechanism: phones are made with a plastic some people are allergic to. That gives a direct biochemical reason telephones could cause facial rashes — supporting the diagnosis. Strengthens.

  2. Strengthens38% picked this

    Contact between other devices and the patients’ faces occurred equally on both sides

    This rules out a competing explanation. If other things touched the patients' faces equally on both sides, those other contacts can't explain why the rash showed up on only one side. That leaves the asymmetric phone contact as a more plausible cause. Strengthens.

  3. Strengthens1% picked this

    Most of the patients had occupations that required them to use

    If most patients had jobs requiring extensive phone use, that's a dose-response point: more exposure to the suspected cause among the affected group strengthens the case that the exposure caused the rash. Strengthens.

  4. Correct60% picked this

    Telephones are used by most people in the

    Why this is right

    This says most people in the industrialized world use telephones — true, but it doesn't help the diagnosis. The fact that telephones are widespread doesn't do anything to link these particular patients' rashes to phone use. If anything, widespread phone use without widespread rash actually suggests phones don't typically cause rashes. Either way, this fact doesn't strengthen the inference that these patients' rashes came from their phones.

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

  5. Strengthens1% picked this

    The complaints occurred after an increase in the patients’ use of

    This establishes timing: rash onset followed an increase in phone use. The cause should precede the effect — and that's exactly what this answer establishes. Strengthens.

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