I also chose options 1, 3, and 4 because together they cover most of the possibilities. SSRIs are not ALWAYS effective in individuals with low serotonin levels. So option 1 is definitely a valid argument but not always the case. Options 3 and 4 are pretty self explanatory.
In particular, I am interested in the structural differences in the brain in those with mental disorders and so-called "normal" people. In my reading, I have found that the amygdala is often cited as being implicated, especially in those with OCD, anxiety disorder, and panic disorder. But I agree with YT2095, a physiological change may not always accompany a mental disorder.
Mokele, you are correct, essentially, every process probably involves some sort of structural change. I guess the difference may be the magnitude of that change. Or the comparison to the "norm". Or maybe not.
Another question I came across during my research (I did a lit review on mental disorders and the brain) is which comes first? Did the structural changes cause the mental disorder, or did the mental disorder cause the structural changes?