If we're not folding, this is how I see it:

Just calling will allow MP to raise with a larger range of hands. I can see him testing BB with his KK, QQ, AQ-AJ, or even less.

However, if we raise here, we can freeze up any tricky stuff from MP and face him with two bets, against the original preflop raiser and the capper. He certainly could only reraise a monster in this instance. If he did I think we fold.

For BB to cap and lead out the flop takes a certain confidence, so our raise not only forces MP to expose his strength, but tests BB as well, while it's still cheap to do so. Without a big ace I don't see how he can 3-bet us given the action.

Overall, I think we can't fold if we might have the best hand here, and our only way of gaining insight to that is to raise for information. Any thoughts?