Murrdox said:If I cast a spell, and provoke an AoO... and the AoO disrupts my spell... I still lose my action. I still TRIED to cast a spell, even though it failed.
Being tripped while standing up from prone does the same thing.
But why did you loose the spell? Did you loose it because the attacker decided to take an AoO? No, you lost it because you lost your concentration (in this case the concentration check was propmpted by the _damage_ that resulted from the AoO but that is besides the point, a Melf's Acid Arrow can also have the same effect). Standing up does not require concentration. Unless the attacker does something else to prevent the defender from standing up just taking damage doesnt' prevent you from taking the Stand Up action.
Ah, just had a thought. The attacker could use his AoO to grapple the defender. Grappling would prevent the defender from completing his action. But a trip doesn't prevent the defender from completing his action after the trip is done.