What incidental anatomical variant is demonstrated here?
Azygos fissure (also called an azygos lobe, although this is a misnomer)
What forms this anomaly?
A (misnamed) azygos lobe is created when a laterally displaced azygos vein makes a deep fissure in the upper part of the lung during embryological development. It is misnamed because it is not a separate lobe.
The heart, mediastinum, lungs and pleura appear normal, other than a line running obliquely through the right upper zone.