The chest X-ray shows.

Right upper lobe collapse. There is a triangular shaped density in the right upper zone with displacement of the horizontal fissure. There is also shift of the trachea to the right with volume loss in the right hemithorax.

In this patient, the cause was a mucus plug in the RUL bronchus. Patient coughed up mucus plug and subsequent x-ray demonstrated resolution.

If the cause of RUL collapse is an endobronchial mass in the upper lobe bronchus, a bulge at the hilum gives an S shape to the inferior aspect of the collapse and this is called the GOLDEN S SIGN.

Reference: WHO Manual of Diagnostic Imaging.