A 36-year-old teacher presents to your clinic, complaining of sharp, knifelike pain on the left side of her chest for the last 2 days. Breathing and lying down make the pain worse, while sitting forward helps her pain. Tylenol and ibuprofen have not helped. Her pain does not radiate to any other area. She denies any upper respiratory or gastrointestinal symptoms. Her past medical history consists of systemic lupus. She is divorced and has one child. She denies any tobacco, alcohol or drug use. Her mother has hypothyroidism and her father has high blood pressure. On examination, you find her to be distressed, leaning over and holding her left arm and hand to her left chest. Her blood pressure is 130/70, her respirations are 12 and her pulse is 90. On auscultation her lung fields have normal breath sounds with no rhonchi, wheezes or crackles. Percussion and palpation are unremarkable. Auscultation of the heart has an S1 and S2 with no S3 or S4. A scratching noise is heard at the lower left sternal border, coincident with systole; leaning forward relieves some of her pain. She is nontender with palpation of the chest wall. What disorder of the chest best describes these symptoms