Chief Complaint: 74-year-old woman with shortness of breath and swelling.

History: Martha Wilmington, a 74-year-old woman with a history of rheumatic fever while in her twenties, presented to her physician with complaints of increasing shortness of breath ("dyspnea") upon exertion. She also noted that the typical swelling she's had in her ankles for years has started to get worse over the past two months, making it especially difficult to get her shoes on toward the end of the day. In the past week, she's had a decreased appetite, some nausea and vomiting, and tenderness in the right upper quadrant of the abdomen.

On physical examination, Martha's jugular veins were noticeably distended. Auscultation of the heart revealed a low-pitched, rumbling systolic murmur, heard best over the left upper sternal border. In addition, she had an extra, "S3" heart sound.
What is causing this murmur?