[DannyTN]

"Right, so why would God create them (chimps, guinea pigs) with a design deficiency (inability to synthesize vitamin C)? "

There is no reason to believe it's a design deficiency.

There are two possibilities:

1. They were designed that way for a reason that we have yet to discover, or
2. They were designed with the ability to synthesize vitamin C and due to the curse on the earth have lost that ability due to mutations.

[VadeRetro]

They were designed that way for a reason that we have yet to discover...

There is no possible mess about which you cannot say the above. Thus, this is useless non-information.

[shubi]

Danny, there are more than two possibilities.

This is the fallacy of False Dilemma, giving two options when there are more than two.