Complex sentence: Most idiopathic cases are diagnosed as "idiopathic pulmonary fibrosis", a diagnosis of exclusion, yet having a characteristic set of histologic/pathologic features known as usual interstitial pneumonia (UIP).
Split into simpler sentences: Most idiopathic cases are diagnosed as "idiopathic pulmonary fibrosis". This is a diagnosis of exclusion of a characteristic set of histologic/pathologic features known as usual interstitial pneumonia (UIP).