Question

A $38$-year-old woman is evaluated in the emergency department for the sudden onset of severe dyspnea and right-sided pleuritic chest pain. She has a history of systemic lupus erythematosus and has experienced two prior first-trimester miscarriages. She currently takes hydroxychloroquine. Her temperature is $37.2^\circ\text{C}$ ($99.0^\circ\text{F}$), blood pressure is $108/64\text{ mmHg}$, pulse is $118/\text{min}$, and respirations are $26/\text{min}$. Pulse oximetry shows $92\%$ on room air. Physical examination reveals clear breath sounds bilaterally. Cardiac examination demonstrates a regular, tachycardic rhythm with a palpable impulse along the left parasternal border. Auscultation reveals a widely split second heart sound with a loud pulmonic component. A high-pitched, blowing holosystolic murmur is heard best at the left lower sternal border. When the patient takes a deep breath in, the intensity of this holosystolic murmur significantly increases. Examination of the lower extremities shows asymmetric right calf swelling with trace pitting edema. Which of the following is the most likely explanation for the auscultatory finding that increases with deep inspiration in this patient?