Cardiac Myxoma

A 45-year-old woman presents with a 3-month history of progressive dyspnea, orthopnea, and intermittent palpitations. On auscultation, a low-pitched early diastolic sound is heard after S2, followed by a mid-diastolic rumble. Transthoracic echocardiography reveals a pedunculated, gelatinous mass arising from the interatrial septum at the fossa ovalis, prolapsing through the mitral valve during diastole. The structure marked **B** in the diagram is most consistent with which diagnosis, and what is the primary mechanism of her obstructive symptoms?