My Daughter Is 18 And A Collegiate Soccer Player, Started

My daughter is 18 and a collegiate soccer player, started pain in left shoulder blade 4 days ago, moved to pain in left chest 2 days ago, trip to ER with severe chest pain upon breathing last night, chest x-ray ok, CT clear, but said pleurisy, slight effusion on left lung - symptoms of dry pleurisy. Now team doctor wants to put her on prednisone for 4 days - why?