Soru

A 72-year-old man presents to the clinic with progressive exertional dyspnea, fatigue, and bilateral lower extremity swelling over the past 6 months. He also reports waking up frequently at night due to numbness and a "pins and needles" sensation in both hands, predominantly affecting the thumbs, index, and middle fingers. His medical history is notable for hypertension, for which he takes lisinopril. Blood pressure is 112/70 mm Hg, and pulse is 84/min. Physical examination reveals jugular venous distention to 9 cm $H_2O$. Lungs are clear to auscultation. Cardiac examination shows a regular rhythm with a prominent S4 gallop; no murmurs are appreciated. Bilateral 2+ pitting edema is present up to the mid-calves.

Electrocardiography demonstrates normal sinus rhythm with first-degree atrioventricular block, low QRS voltage in the limb leads, and QS waves in leads V1 to V3.

Transthoracic echocardiography reveals a left ventricular ejection fraction of 55%, severe concentric left ventricular hypertrophy (interventricular septum thickness of 1.7 cm) with a "sparkling" granular appearance of the myocardium, and severe biatrial enlargement. Doppler evaluation shows a restrictive filling pattern.

Which of the following is the most appropriate next step in the diagnostic evaluation of this patient?