する verb
しんだんする
その医者が私に癌だと診断するほんの少し前に、ある看護婦さんが私の事をちらっと振り返って見たんです。
Right before the doctor diagnosed me with cancer, a nurse glanced over her shoulder at me.
診断 is a diagnosis. Make this into a verb, and you have to diagnose.
This is a jukugo word, which usually means on'yomi readings from the kanji. If you know the readings of your kanji you'll know how to read this as well.