Q. A 42-year-old woman comes to the clinic because of a 12-month history of difficulty swallowing liquids and solids. She describes episodes of food "sticking in her chest" and regurgitating undigested food. She has lost a few pounds over the last year. Past medical history is unremarkable, she takes medications, and reports that she is otherwise very healthy. Family history is unremarkable. Temperature is 36.7 (98.7 F), Blood Pressure is 130/80, Heart Rate is 72 beats/minute. Neurological examination is unremarkable. Cardiopulmonary auscultation is normal except for some mild crackles at the lung bases. No masses present on abdominal examination. Esophageal barium swallow reveals proximal esophageal dilation and distal constriction (see image). After upper endoscopy is performed and is negative for any evidence of malignancy. You offer a presumptive diagnosis to the patient but she asks for confirmation. Which of the following is the gold standard to diagnose this disorder?