54-year-old man presents with progressive shortness of breath, lower extremity edema unresponsive to diuretic therapy, and fatigue upon exertion. His past medical history is significant for hypertension and he is a former smoker. On examination, his temperature is 37 degrees C; respiratory rate is 18 breaths per minute; and his BMI is 26 kg/m2. His lungs are clear. Heart acultation reveals normal S1, physiologically split S2, accentuated P2, 3/6 holosystolic murmur on the left upper sternal border that increases with inspiration, and 2/6 diastolic murmur on pulmonic area. His liver is palpable by 2 fingers. There is pitting edema in his lower extremities. The rest of his physical examination is normal. What diagnostic test will be most eful in the evaluation of the etiology of this patient's chief complaints?