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Pulmonary Apical Cap as a Potential Risk Factor for Pleuroparenchymal Fibroelastosis. Pleuroparenchymal fibroelastosis (PPFE) is a progressive and frequently fatal interstitial lung disease that involves the upper lobes. Although its cause remains unknown, the histopathologic evidence underlying PPFE bears striking resemblance to that of the pulmonary apical cap (PAC), a relatively common and benign entity. We describe the case of a patient with PAC that evolved into distinctly asymmetric PPFE over 6 years after unilateral surgical lung injury. Given the histologic similarity between these two conditions, we propose that these two entities underlie common biologic pathways of abnormal response to lung injury, with the presence of a PAC increasing susceptibility to the development of PPFE in the face of ongoing inflammatory insults. This case describes the histopathologic evolution of PAC to PPFE before and after an inciting injury.