Afghanistan Comprehensive Specialization Exam Practice Questions - Afghanistan Comprehensive Specialization Completion Exam (Takhasus) Exam

A 74-year-old male with a history of bilateral carpal tunnel syndrome and lumbar spinal stenosis presents with progressive exertional dyspnea, orthopnea, and lower extremity edema over the past 8 months. 12-lead ECG demonstrates normal sinus rhythm with low QRS voltage in limb leads and pseudo-infarction QS waves in leads V1–V3. Echocardiography reveals marked symmetric concentric left ventricular wall thickening (septum 18 mm), biatrial enlargement, mildly reduced LVEF (42%), and speckle-tracking global longitudinal strain showing preserved apical strain with severe basal and mid-ventricular impairment ('apical sparing' pattern). Serum and urine protein electrophoresis with immunofixation and serum free light chain (sFLC) ratio are completely normal. What is the next most definitive non-invasive diagnostic investigation to confirm the underlying etiology?