The case of the missing right heart border

A 27 year old female presented to the ED complaining of sharp right sided chest
pain which was pleuritic in nature. There was no history of trauma. She was not
breathless. She was systemically well with no cough, sputum or
haemoptysis.  She had no signs or symptoms suggestive of PE and no VTE risk factors. She was not tachycardic or hypoxic and an ECG was completely normal.
I was anticipating a normal CXR and was surprised by what I saw:

What is your suspected diagnosis based on the CXR?

There is apparent loss of the right heart border which we normally associate with right middle lobe consolidation, but can also be seen with pectus excavatum. This was confirmed on a lateral CXR.
Interestingly she had a normal looking chest on physical examination.
Here's a nice refresher on cardiac border abnormalities on CXR.

Thanks to Beth for the interesting case.  Please feel free to contribute to the blog with any unusual or important findings by chatting to/emailing me.  As always, comments below please!  Si.