Sentence Correction

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

Answer is c.

I got it through the process of elimination. My only question is the explanation for there is one leaves it without a referent. I dont understand this explanation. Help?

I don't understand your question nor the explanation you posted which seems incomplete. "C" is gramatically correct and better as a solution because it's the only solution that correctly refers to the condition ("it") throughout with the appropriate verbs and pronoun references.

The explanation in the official guide for choice a and b states "for there is one leaves it without a referent". I dont understand this explanation.

I see...what they are saying is that when the sentence says "there is one" it's not referring to anything in the sentence...(there is one what?) the correct solution uses "it" to refer correctly to the condition.