What are the chances.......

That this daily run of the mill random patient that you  see in the ER with all the pattern of symptoms that have become second nature to you to slot and eal with, is an initial manifestation of a rare syndrome that you existed only in the small paragraphs of the end of the chapter in Harrison that you never read? 

That would depend on the questions you ask I guess. No one comes to the doctor saying I have Langerhans cell histiocytosis or MELAS. You could be very comfortable treating these guys as "COPD exacerbation" or "young stroke with negative work up" and never know for once whats going on. It runs well with your spiel of things- protocol done, rehab worked out, patient feels well, D/C to nursing home. 

You could very well put it the other way around and ask, well......WHAT ARE the  chances?

Well.....it all depends on the questions you ask.