The brains of identical twins are genetically identical. When only one

Conclusion:
damage to physical structure -> schizophrenia. [Cause -> Effect]

(B) The relative smallness of certain parts of the brains of schizophrenics is not the result of schizophrenia or of medications used in its treatment.
Negating B would find an alternative cause for Schi..
schizophrenia or of medications used -> smallness of certain parts of the brains of schizophrenics
Weakens the conclusion.
So B is the answer.

Typical GMAT question...

Conclusion : A(smallness in brain size/Damage in brain) causes B(schizophrenic) ...

so we need to discard the possibility that B(schizophrenic) causes A(smallness in brain/Damage in brain)

B is doing that perfectly ...........

Physical damage/Relative smallness in the size of the brain causes Schizophrenia.
Assumption - Schizophrenia doesn't cause physical damage to the brain.

A - This is already mentioned in the argument.
B - This looks likely.
C - Irrelevant
D - That's not what were are trying to derive here. Irrelevant
E - Irrelevant.

B is the answer IMO

let us take a look at the main conclusion --

schizophrenia is caused by damage to the physical structure of the brain.

this is a cause and effect statement. We can write this as --

damage to the physical structure of the brain --> schizophrenia

One of the simplest ways to approach an assumption question would be to figure out a flaw in the argument. Negating that flaw would lead us to an assumption made in the argument.
Commonly found patterns of flaws in causal statements are reverse causation (Effect --> Cause) and alternate cause (Another cause --> effect).

let us take a look at the answer options --

Option A - Incorrect
Not a proper comparison. We are comparing the brains of IDENTICAL TWINS -- from this we are deriving our conclusion that physical damage to the brain causes schizophrenia.

Option B - Incorrect
If the smallness of the brain in the identical schizophrenic twin is due to "medications", this destroys the argument by providing an alternate cause.
If it is due to schizophrenia itself, this tells us that the effect led to the cause (Reverse causation) -- again weakens the argument.
By denying both these flaws, option B states an assumption.

Option C - Incorrect
Not a proper comparison. We are comparing the brains of IDENTICAL TWINS -- this helps us to control for other factors that might impact the brain size. We are not comparing the brain sizes of twins to those of other people.

Option D - Incorrect
this directly contradicts the conclusion -- which says that different size (smaller brain size due to physical damage) is something that causes schizophrenia

Option E - Incorrect
Not relevant to the argument. the argument is concerned with WHAT CAUSES schizophrenia, not HOW LIKELY it is to occur.

Only B and D stand a chance here. D loses out to B since negating it doesn't break the passage apart. After negation D looks like
"When a pair of identical twins both suffer from schizophrenia, their brains are not the same size."
This becomes irrelevant.

B tests the A ---> B or B ---> A principle.

Answer B.

(B) The relative smallness of certain parts of the brains of schizophrenics is not the result of schizophrenia or of medications used in its treatment.
Thsi matches with our pre thinking no (ii)
If we negate it what will happen
just check
The relative smallness of certain parts of the brains of schizophrenics is the result of schizophrenia or of medications used in its treatment

See the negated version shatters conclusion

(C) The brain of a person with an identical twin is no smaller, on average, than the brain of person who is not twines.out of scope

(D) When a pair of identical twins both suffer from schizophrenia, their brains are the same size stimulus never talks about this so out of scope

(E) People who have an identical twin are no more likely to suffer from schizophrenia than those who do not.
irrelevant
Hence B