The correct answer is- had spread. Key Points ​We have to use the past perfect tense in the underlined part of the sentence. It is used to make it clear that one event happened before another in the past. It does not matter which event is mentioned first - the tense makes it clear which one happened first. In the given question, the doctors did the X-ray first, this is the first event. Then they found that the other kidney was affected by the infection, this is the second event. The simple past will be used for one event and past perfect will be used for the other event. Eg: I had stepped out when the phone rang. ​ Correct Sentence: The doctors did another X-ray this morning and found that the infection had spread to the other kidney. ​​​​Additional Information The past perfect refers to a time earlier than before now.