Valvular Heart Disease USMLE Step 1 Practice Question

A 32-year-old woman presents to clinic with progressive dyspnea on exertion and fatigue over the past 2 years. She has a history of acute rheumatic fever at age 8 and reports being told she had a heart murmur as a child but has not had cardiac follow-up in over a decade. On physical examination, her vital signs are: BP 118/76 mmHg, HR 88 bpm, RR 18, O2 sat 96% on room air. Cardiac auscultation reveals a loud S1, an opening snap in early diastole, and a low-pitched, rumbling diastolic murmur best heard at the apex with the patient in the left lateral decubitus position. The murmur does not radiate to the axilla. Transthoracic echocardiography demonstrates a mitral valve area of 1.2 cm² (normal >4 cm²), severe left atrial enlargement, and normal left ventricular systolic function. Which of the following best explains the hemodynamic abnormality responsible for this patient's symptoms?