Multi-hit spell - Multiple smaller Concentration checks or one larger one?

Question

Some spells like magic missile and scorching ray can strike a target multiple times. I recently had cause to consider using a damage spell to disrupt an opponent's spellcasting - and realized I'm not sure how multi-hit spells and concentration checks interact.

How many Concentration checks should a spellcaster make if he's struck with multiple hits from a single spell during spellcasting? Should the damage from the spell be totaled for one big check? Or should each hit force a separate check, giving multiple chances to roll low?

PFSRD: Magic chapter, Concentration heading, Spell sub-heading:

If you are affected by a spell while attempting to cast a spell of your own, you must make a concentration check or lose the spell you are casting. If the spell affecting you deals damage, the DC is 10 + the damage taken + the level of the spell you're casting.

This particular rule may complicate things, or simplify them, depending. It might seem to override the Injury sub-heading we're probably all already familiar with.

The accepted answer will probably be the highest answer in this hierarchy:

book text
official developer clarification
developer commentary
answer with some support from the above
answer with a clear consensus
other

Answer 1

Each Source of damage counts for a new concentration check, and also counts as a single attack for anything that would affect a magic attack, like precision damage.

An example of such interaction is Ice Storm, which causes 3d6 bludgeoning damage and 2d6 points of frost damage. You should sum the damage of the two types (apply any frost resistance before you do so), as they come from the same source of damage, then roll your concentration check.

While other spells, nearly all the ones that require multiple attack rolls, will count each hit as a different attack, thus they are different sources, such as Scorching Ray.

So, if you are attacked with a Ice Storm spell, you should roll your concentration once against DC 10 + total damage taken. However, if you are hit by a Scorching Ray spell, you should roll 10 + damage taken once per attack roll against you that caused damage.

Please note with this that multiple sources of damage will never stack when you calculate the concentration check DC.