Agrandmother brings her 13-year-old grandson for evaluation. she noticed last week when he took off his shirt that his breastbone seemed collapsed. he seems embarrassed and says that it has been that way for awhile. he states he has no symptoms from it and that he just tries not to take off his shirt in front of anyone. he denies any shortness of breath, chest pain, or lightheadedness on exertion. his past medical history is unremarkable. he is in sixth grade and just moved in with his grandmother after his father was transferred for a work contract. his mother died several years ago in a car accident. he states that he does not smoke and has never touched alcohol. examination shows a teenage boy appearing his stated age. visual examination of his chest reveals that the lower portion of the sternum is depressed. auscultation of the lungs and heart is unremarkable. what disorder of the thorax best describes these findings?