LSAT 158 | Section 4 | Logical Reasoning: Q13

LR Question 13 Explanation

QUESTION TEXT: Medical researcher: A new screening test detects certain polyps…

QUESTION TYPE: Necessary Assumption

CONCLUSION: The new test can lead to unnecessary, dangerous operations.

REASONING: The new test detects polyps that might not be malignant. Doctors remove polyps even if they aren’t malignant.

ANALYSIS: This feels like a good argument. When this happens on a necessary assumption question, look for a term in the conclusion that wasn’t mentioned in the stimulus. Here, the conclusion says the operations are unnecessary. Why? Because the polyps aren’t malignant.

But hold on: those aren’t the same things. The fact that a polyp isn’t malignant doesn’t necessarily mean it’s ok to leave the polyp in the body. Maybe it has other bad effects or could grow to become malignant later. The argument is assuming none of this true and the polyp removal is indeed unnecessary.

So as a prephrase, look for necessity. Only one answer does, so this kind of prephrase can help you go through answers very fast.

(In a medical context, a malignant growth generally means cancerous. But some non-malignant growths, for example non cancerous brain tumours, nonetheless need removal.)

___________

1. We don’t care about what is ethically justified. This argument is factual, not moral.
2. CORRECT. This matches our prephrase. If you negate this, the argument falls apart.
   Negation: It is always medically necessary to surgically remove nonmalignant polyps.
3. This answer choice tells us what we should/shouldn’t do. This is incorrect: the LSAT makes a strict separation between facts and morals, and this question is a factual question.
4. Like A and C, this answer is moral. It tells us what we should do, whereas the argument is about what is dangerous. You cannot switch from morals to facts on the LSAT!
5. Do we know anything about “medically useful”? No! Eliminate. The test could be useful for giving all clear: negative results are often useful.

Recap: The question begins with “Medical researcher: A new screening test detects certain polyps”. It is a Necessary Assumption question. Learn how to master LSAT Necessary questions on the LSAT Logical Reasoning question types page.