A 32-year-old woman presents with palpitations and shortness of breath after activity for over 3 years, inability to lie flat at night, and coughing up pink frothy sputum for 1 hour. One week ago, she had fever, sore throat, and cough. Respiratory rate is 30/min, bilateral lungs show diffuse dry and wet rales/crackles, cardiac borders are enlarged bilaterally, and a grade 2/6 systolic blowing murmur is heard at the apex. The most likely diagnosis is: