QUESTION TEXT: Microbiologist: Because heavy metals are normally concentrated…
QUESTION TYPE: Strengthen
CONCLUSION: Heavy metals in sludge make bacteria resistant to antibiotics.
REASONING: Heavy metals made bacteria resistant to heavy-metal poisoning.
ANALYSIS: This is a causation-correlation error. Humans brains (including yours) are wired to misunderstand correlations. We see that two things happen and assume that one causes the other. So the LSAC tests us on this quite a bit. But once you learn to recognize the error, the questions aren’t hard. They usually don’t add additional tricks. So remember: the fact that two things happen together doesn’t mean one causes the other.
You can strengthen a correlation/causation relationship by ruling out alternate explanations. You may know in real life that sewage is full of antibiotics. This is a plausible way to cause antibiotic resistance. You can strengthen the argument by ruling out this possibility. You’re definitely allowed to use outside knowledge to form hypotheses, as I just did.
- So? Thankfully, most bacteria aren’t antibiotic resistant. And the stimulus said heavy-metal resistance is unusual. So we shouldn’t be surprised that most bacteria are vulnerable to both antibiotics and heavy metals.
- CORRECT. This amounts to a controlled experiment. Everything else is kept equal, except for heavy metals. And bacteria don’t become antibiotic resistant. So it must be the heavy metals that cause the resistance to antibiotics.
- This weakens the argument. It shows that the bacteria were likely antibiotic resistant already. They developed heavy-metal resistance as a result.
- This weakens the argument. It could be the antibiotics that cause antibiotic resistance.
- So? This is vague. Many could be “150 kinds of bacteria out of 10 trillion total”. And why are they resistant to both heavy metals and antibiotics? This does nothing to help prove that heavy-metals lead to antibiotic resistance.
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