A 31-year-old woman, gravida 3, para 2, who is at 32 weeks' gestation, is
admitted to the hospital because of a 1-week history of progressive
dyspnea and wheezing. She says her heart is "racing" and she is coughing
up a small amount of blood-streaked sputum. Medical history is
significant for hypothyroidism, for which she takes levothyroxine. An
intravenous catheter is placed. Vital signs are temperature 36.9°C (98.4°F),
pulse 132/min, respirations 32/min, and blood pressure 135/78 mm Hg.
Pulse oximetry on 100% oxygen via nasal cannula shows an oxygen
saturation of 92%. Auscultation of the lungs discloses decreased breath
sounds at the bases with expiratory crackles bilaterally. Cardiac
examination discloses an irregularly irregular rhythm, an indistinct point of
maximal impulse, and a loud S1. A grade 3/6, low-pitched, diastolic,
rumbling murmur is audible at the apex; a distinct snapping sound
precedes the murmur. Fetal heart rate is 144/min.
Which of the following is the most likely cause of the findings in this
patient?
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