Any medical test will sometimes fail to detect a condition when it is present and indicate that there is one when it is not.

(A) a condition when it is present and indicate that there is one
(B) when a condition is present and indicate that there is one
(C) a condition when it is present and indicate that it is present
(D) when a condition is present and indicate its presence
(E) the presence of a condition when it is there and indicate its presence

OA: C

I understand the OA but I don't understand the explanation that OG gives for choice A.

It states "The construction there is one leaves it in the final clause without a referent.

Someone please decipher what it means.