The frontal chest x-ray shows an incidental finding – a right sided aortic arch.
The aortic knob is absent on the left side, instead visible on the right side of trachea with a focal leftward indentation on the trachea.
The anomaly was an incidental finding in the above adult. However, right sided aortic arch can be associated with congenital heart disease. Also, anomalous course of major neck vessels arising from the aorta can cause compression on the trachea or oesophagus leading to breathing difficulties & dysphagia.
Further reading – https://pubs.rsna.org/doi/10.1148/rg.2017160033