The chest x-ray reveals an azygos lobe fissure, an accessory fissure in the right lung. The rest of the lung fields are normal.

Azygos lobe is seen in 1 % of the population, encased in the azygos fissure which contains azygos vein at its lower end. This accessory fissure results from failure of normal migration of the azygos vein from the chest wall to its normal position in the tracheobronchial angle and persistence of the invaginated visceral and parietal pleurae.

Reference: Grainger & Allison’s Diagnostic Radiology, A textbook of Medical Imaging, 6th Edition.