The chest x-ray shows cardiomegaly & 3rd Mogul Sign due to enlargement of the left atrial appendage. There is also evidence of splaying of the carinal angle (LA enlargement) & dilated pulmonary arteries (pulmonary hypertension). Underlying pathology in the patient was presence of severe mitral stenosis.

Normally, the left heart border just below the left main PA should be slightly concave or flat. A convex appearance is due to LA appendage enlargement & is called 3rd Mogul Sign.

Further reading –  https://www.researchgate.net/publication/317999648_Rheumatic_Giant_Left_Atrium-An_Overview