A 68-year-old man presents to the clinic complaining of worsening exertional dyspnea. He feels short of breath even after walking with his dog more than a couple of blocks from his home. He does not have chest pain at rest but has experienced retrosternal chest pressure with strenuous exertion. He admits that he passed out a week ago for unclear reasons. He wakes up at night feeling short of breath. He denies any significant medical history. He has not seen a doctor in 25 years. On physical examination, he is afebrile, with a heart rate of 88 beats per minute (bpm), blood pressure of 140/95 mm Hg, and respiratory rate of 18 breaths per minute. Examination of the head and neck reveals distended neck veins. On cardiac examination, you detect a soft S2 and a coarse, late-peaking crescendo-decrescendo, systolic ejection murmur heard best at the aortic area of the heart. You suspected a cardiac disorder in this patient. Which of the following is the best initial diagnostic modality to confirm the diagnosis?
