Choice E, mitral stenosis, is the most likely finding in this patient, who presents with physical exam findings including a possible opening snap, loud S1, and a very soft diastolic rumbling murmur which is heard best at the cardiac apex and accentuated by placing the patient in the left lateral decubitus position. Although rheumatic fever was not positively confirmed, the patient likely did have a history, given that the majority of cases of mitral stenosis are secondary to rheumatic heart disease. Patients from Asia, Central America, and South America are exposed more frequently than their counterparts in more developed countries, where antibiotic use is more common. Choices A and B, tricuspid regurgitation and tricuspid stenosis, are also linked with patients with rheumatic heart disease. The murmur of tricuspid regurgitation, however, is a systolic murmur, which increases with inspiration and is heard best at the left lower sternal border. Tricuspid stenosis presents with a diastolic murmur, and with inspiration the murmur increases. It, too, is heard best at the left lower sternal border. Choice C, an atrial septal defect, if large, could present with similar symptoms of exertional dyspnea secondary to a large shunt, but auscultation would reveal a moderately loud systolic ejection murmur that is heard best in the second and third interspaces. This is secondary to increased pulmonary arterial flow. Choice D, aortic regurgitation/insufficiency, is also a diastolic murmur; however, it is usually a diastolic decrescendo murmur that is heard best at the left sternal border.