OG中有一句话看不懂

大家好，OG，83题的原题我看了半天楞是不明白说的什么意思。

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

答案是C，但整个句子怎么翻译呢？希望高手给个指点，先谢拉。

噢，原来是这样，谢谢了兄弟 ：）