A 16-year-old girl comes to the hospital with chest pain and respiratory distress. On physical examination, the patient is sho of breath, wheezing and gasping for air. A prominent pansystolic hea murmur and a prominent third hea sound are heard on cardiac auscultation. An X-ray study of the chest shows marked enlargement of the hea. The patient expires despite intense suppoive measures. At autopsy, microscopic examination of the myocardium discloses aggregates of mononuclear cells arranged around centrally located deposits of eosinophilic collagen. What is the appropriate diagnosis?