Logical reasoning PrepTest 158 · Section 4 · Question 13

This doesn't strengthen the argument. So, it isn't necessary for the argument, either.

To get a better sense of why this answer choice doesn't strengthen the argument, let's take the flipside of it. We'll translate it to, "If a dangerous surgical operation does not treat a more dangerous medical condition, the operation is not medically justified."

This doesn't strengthen the argument because we don't know whether the procedure to remove a polyp is more dangerous than the malignant polyp itself. If a premise established that it was, this answer choice would strengthen this argument. But without this key piece of information, we can't say that this would strengthen the argument. And without knowing whether removing polyps poses a greater danger than a malignant polyp, this information isn't necessary, either.

Finally, this is the argument we'd make if we used the backup plan (note that we used the "if-then" translation of the answer choice and negated that statement):

"Doctors can now detect polyps before they know whether the polyps are malignant. Doctors tend to remove these polyps. Removing polyps is dangerous. Some removed polyps are not dangerous. But, even if a dangerous surgical operation does not treat a more dangerous medical condition, the operation can still be medically justified. So, these dangerous operations may be medically necessary."

This doesn't make a lot of sense. We don't have any premises about a "more dangerous medical condition" or a "more dangerous medical operation," so it's unclear how the "But" statement undermines what comes before it.

No matter which approach we use, we can confidently cross off (A).

Argument or Facts:
Argument

Valid or Flawed:
Flawed

Question Type:
Strengthen with a Necessary Premise

Passage Summary:
Doctors can now detect polyps before they know whether the polyps are malignant. Doctors tend to remove these polyps. Removing polyps is dangerous. Some removed polyps are not malignant. So, doctors are performing dangerous operations that are not medically necessary.

Strategy Overview:

1. Argument or facts? Always argument, so identify premises and conclusions of argument
2. Anticipate why the premises are not enough to show that the conclusion is true
3. Use anticipations to select the answer choice that:
1. Strengthens the conclusion of the argument, and
2. Is necessary for the conclusion of the argument
1. Can use "negation test" to double-check: if this answer choice isn't true, does the conclusion become untrue as well? If so, it's necessary for the conclusion of the argument, and it is the correct answer

Answer Anticipation:
On Strengthen with a Necessary Premise questions, it's our job to find the answer choice that (a) strengthens the argument's conclusion and (b) must be true to draw the conclusion. At first, finding an answer that matches both criteria is very challenging for many test-takers.

But with much practice, experienced LSATers develop a reliable strategy. All Strengthen with a Necessary Premise questions feature flawed arguments, and it's much easier to find the correct answer after identifying why the argument is flawed. After all, fixing that flaw will strengthen the argument, and showing that the flaw is fixable is necessary to draw the conclusion.

For most Strengthen with a Necessary Premise questions, the flaw involves introducing a new, important idea in the conclusion, creating a gap between the premises and conclusion. Which part of this conclusion expresses a new idea? What about the part that says removing polyps is "not medically necessary"? Did any premises tell us which operations are and are not medically necessary? Nope. So, we can't say for sure that removing polyps is not medically necessary, even if the polyps turn out not to be malignant and the procedure is dangerous.

The correct answer will probably bridge this gap by connecting one of the premises to this new idea. The correct answer could be as simple as, "Removing polyps that aren't malignant is not a medically necessary procedure." This strengthens the argument (it establishes that the procedures the premises discuss aren't necessary) and is necessary to the researcher's argument (if these procedures were medically necessary, the researcher's conclusion would be invalid).

Finally, many test-takers find the backup plan helpful on these questions. To resort to the backup plan, we'll re-read the premises. Then, we'll say, "But, it's not true that," and read the answer choice. (Or, we can just say "But" and then read the negated form of the answer choice.) Finally, we'll say, "So, it may not be true that ..." and re-read this argument's conclusion. (Or, we can just say "So" and then read the negated form of the conclusion.) If that argument makes sense, we'll know the answer choice expresses a necessary assumption. If the argument doesn't make much sense, then we can cross that answer choice off. Here's a simplified template we can use for this backup plan (note that we simplified the premises and conclusion and included the negated form of the conclusion):

"Doctors can now detect polyps before they know whether the polyps are malignant.

"Doctors tend to remove these polyps.

"Removing polyps is dangerous.

"Some removed polyps are not dangerous.

"But, it's not true that [answer choice].

"So, these dangerous operations may be medically necessary."

Answer Choice Explanation:
As we anticipated, this answer choice strengthens the argument by telling us removing nonmalignant polyps is not always medically necessary. So, this answer choice establishes that some doctors perform dangerous but medically unnecessary operations when they remove nonmalignant tutors. This strengthens the researcher's argument, making us more confident that this new screening test prompts dangerous but medically unnecessary operations.

We also need this information to conclude that the new screening test prompts dangerous but medically unnecessary operations. If removing nonmalignant polyps was always medically necessary, we'd have no evidence that doctors perform dangerous but medically unnecessary operations. So, we need to know that some of these operations on nonmalignant polyps are not medically necessary.

We can also evaluate this answer choice using the backup plan. Here's the argument we'd make if we used the backup plan (note that we simplified and negated this answer choice to make the argument easier to read):

"Doctors can now detect polyps before they know whether the polyps are malignant. Doctors tend to remove these polyps. Removing polyps is dangerous. Some removed polyps are not dangerous. But removing nonmalignant polyps is always medically necessary. So, these dangerous operations may be medically necessary."

This makes sense! Yes, we're removing nonmalignant polyps in a dangerous operation. But removing such polyps is medically necessary, so these dangerous operations might be medically necessary. That adds up and sounds like a cogent paragraph.

Therefore, no matter which approach we used, we can be confident that (B) is the correct answer.

Key Takeaways:

For most Strengthen with a Necessary Premise questions, the flaw involves introducing a new, important idea in the conclusion, creating a gap between the premises and conclusion.

This doesn't strengthen the argument. That means it isn't necessary for the argument, either.

This doesn't strengthen the argument because the argument doesn't conclude that we should not use the new polyp screening test or that we should change the test. So, the "then" part of this answer choice is irrelevant to the conclusion.

Plus, we don't know that these new screening tests encourage "medically unnecessary operations." The researcher concludes that these operations are medically unnecessary. But, as we discussed in the Answer Anticipation section, this conclusion is flawed. The premises didn't tell us which kinds of operations are not medically necessary. Since we don't know whether these tests are medically unnecessary this answer choice might not even apply to the argument. Put another way, when an argument in a Strengthen or Weaken question concludes "X," the correct answer won't begin with "If X." The argument didn't prove that "X" is true, so the claim that begins "if X" is true won't add anything to the argument.

Finally, this is the argument we'd make if we used the backup plan (note that we simplified and negated the answer choice to make the argument easier to read):

"Doctors can now detect polyps before they know whether the polyps are malignant. Doctors tend to remove these polyps. Removing polyps is dangerous. Some removed polyps are not dangerous. But, even if a new screening test encourages medically unnecessary operations, we can still use the test and not modify it. So, these dangerous operations may be medically necessary."

This doesn't make a lot of sense. The part about modifying or not using the test doesn't relate to anything that came before it, so it's unclear how the "But" statement contradicts the previous premises. And the "But" statement discusses medically unnecessary operations, so it doesn't provide good support to a conclusion about medically necessary operations.

No matter which approach we use, we can confidently cross off (C).

This doesn't strengthen the argument. That means it isn't necessary for the argument, either.

This doesn't strengthen the argument because the argument isn't about removing malignant tumors. The argument is only concerned with whether removing nonmalignant tutors is medically unnecessary. Whether doctors should remove malignant tumors doesn't help us establish that removing nonmalignant tumors is medically unnecessary.

Alternatively, if we used the backup plan, this is the argument we'd make (note that we simplified and negated the answer choice to make the argument easier to read):

"Doctors can now detect polyps before they know whether the polyps are malignant. Doctors tend to remove these polyps. Removing polyps is dangerous. Some removed polyps are not dangerous. But, even if a malignant polyp is detected, it should not necessarily be surgically removed. So, these dangerous operations may be medically necessary."

This doesn't make any sense sense. The "But" statement is about malignant polyps, not nonmilignant polyps. Plus, if that statement suggests that we don't have to remove all malignant polyps, that might weaken the conclusion removing nonmalignant polyps may be medically necessary. This argument is a real stinker.

No matter which approach we use, we can confidently cross off (D).

This doesn't strengthen the argument. That means it isn't necessary for the argument, either.

To get a better sense of why this answer choice doesn't strengthen the argument, let's take the flipside of this statement. We'll translate it to, "If a screening test does not detect a polyp that requires treatment, then the screening test is not medically useful."

This doesn't strengthen the argument because the conclusion doesn't assert that the screening test is not medically useful. The conclusion asserts that the operation is not medically necessary. These are two different concepts. Even if the test isn't medically useful, the operation the test prompts might be medically necessary.

Alternatively, if we used the backup plan, this is the argument we could construct (note that we used the "if-then" translation of the answer choice and negated it to make the argument easier to read):

"Doctors can now detect polyps before they know whether the polyps are malignant. Doctors tend to remove these polyps. Removing polyps is dangerous. Some removed polyps are not dangerous. But, even if a screening test does not detect a polyp that requires treatment, then the screening test can still be medically useful. So, these dangerous operations may be medically necessary."

This doesn't make any sense sense. The "But" statement suggests that the screening test for polyps may be medically useful. But that doesn't mean the dangerous opearations are necessary. The screening tests could be useful for other reasons.

No matter which approach we use, we can confidently cross off (E).