A 35-year-old woman with a 10-year history of rheumatic mitral stenosis (MS) presents to the emergency department with sudden-onset dyspnea and palpitations for 1 hour after an argument. Examination reveals: sitting upright, cyanosis of the lips, restlessness, respiratory rate 40/min, heart rate 150/min with regular rhythm. Acute pulmonary edema is suspected. Which clinical manifestation is most specific for the diagnosis?