He won't release his tax returns, so we have to speculate.
Today's guess: medical expenses.
Medical expenses would have to be rather large for Mitt to declare them ... unless of course he had driven his taxable income down via the various methods available to a 0.0001%er.
So if he ever took medical expenses as a tax deduction, it wasn't for putting braces on Tagg. It was for something serious. Something serious enough that voters would care about his ability to serve out his term as President, or that would impair his ability to do his job.
Is Mitt HIV positive?
Did he have a heart attack, or cancer?
Has he ever been treated for mental illness or a (lack of) personality disorder?
We don't know. And we can't know until the returns are released.