I'd go with the above.
I had a strange USMLE problem. In my class I was one of the best rated psychiatry students. I did one of the best in terms of scores etc.
but in my USMLE2; I got obliterrated in my scoring for Psychiatry-putting me in one of the lowest tiers for that area. The only theory I could come up with was that I knew too much and started questioning the questions too much. Strangely I did best on my Pulmonogy section and I felt that was an area I was not well prepared putting me in one of the best tiers for that.
Anyways, the USMLE answer is probably the above, but in reality---
It could be antidepressant induced. SNRIs tend to do it more than SSRIs. The theory is the more pathways involved, the more likely it'll put you into mania. E.g. TCAs more so than SNRIs more so than SSRIs (just to inform you I heard this anecdotally but I did read studies confirming that effexor (an SNRI) does it more often than SSRIs).
However, strangely, Welbutrin does it much less often than any of the above and it works on 3 known pathways (Dopamine, Norepinephrine & serotonin).
If I had a pt who was induced into mania due to an antidepressand & that patiient was diagnosed with MDD, I would consider that perhaps the MDD was a misdiagnosis. Its possble. But it after looking into it and confirming it with collateral information that the patient never had a previous manic episode, and ruling out other causes such as illicit substance use, I would then chalk it up to substance induced mood d/o, in which case I'd try to lower the dose of the antidepressant or switch it to another antidpepressant, possibly Welbutrin (only if its the right choice), because Welbutrin does it much less often compared to the other antidepressants around.
If it did turn out that the pt had a previous manic episode or hypomania, I would change the diagnosis to Bipolar I or II depending on what the previous info turned up then change the pt's medication to a mood stabilizer--probably Lamictal if the pt had more depression than mania because lamictal has some evidence that it pushes people out of depression better than the other mood stabilizers.