Complete Question Explanation
Assumption-SN. The correct answer choice is (A)
The argument in this stimulus is basically that if a mental disorder results from a chemical imbalance, it can only be effectively treated with medication that will reduce or correct the chemical imbalance. The unstated presupposition here is that other forms of treatment (psychotherapy, say) cannot correct the chemical imbalance. In order to make the argument work, we need to find an answer choice that provides this missing assumption.
Answer choice (A): This is the correct answer choice. Assume the logical opposite of this statement: What if psychotherapy can produce effective reduction/correction of chemical imbalances that cause mental disorders? If this were true, then the author's argument fails — medication would not be the only way to treat chemically-caused disorders. So by the Assumption Negation test, this statement is necessary for the argument to work.
Answer choice (B): The argument makes no reference to the speed of results; it discusses only the effectiveness of treatment. So this answer choice is irrelevant.
Answer choice (C): We do not need to assume that most mental disorders have non-chemical causes. Even if most mental disorders do have chemical causes, the argument can still work — we would just be talking about a very large number of disorders.
Answer choice (D): Assume the logical opposite of this statement: What if psychotherapy were sometimes more effective than medication in treating patients with mental disorders? This deals generally with all mental disorders of various causes. This would not undermine the specific argument about medication being the only effective treatment for chemically-caused mental disorders. Thus, by the Assumption Negation test, this answer choice is not necessary for the argument.
Answer choice (E):"Effective treatment" was not defined in the stimulus. Does effective treatment imply merely a reduction of symptoms, or a deep correction of the underlying cause? This is not made clear. For this reason alone, we could reject this answer choice. Additionally, try the Assumption Negation test: Ask yourself, what if psychotherapy has an effect on mental disorders beyond just reduction of symptoms? Just like answer choice (D), this is a very general statement about all mental disorders, and it does not defeat the specific claim about medication being the only effective treatment for chemically-caused mental disorders.