A businessman who presented with substernal heaburn is diagnosed having gastroesophageal reflux disease (GERD). BEST test to diagnose GERD and quantify acid output is:

The most sensitive test for diagnosis of GERD is 24-h ambulatory pH monitoring. Endoscopy is indicated in patients with reflux symptoms refractory to antisecretory therapy; in those with alarming symptoms such as dysphagia, weight loss, or gastrointestinal bleeding; and in those with recurrent dyspepsia after treatment that is not clearly due to reflux on clinical grounds alone. Ref: Wong-Kee-Song L.M., Topazian M. (2012). Chapter 291. Gastrointestinal Endoscopy. In D.L. Longo, A.S. Fauci, D.L. Kasper, S.L. Hauser, J.L. Jameson, J. Loscalzo (Eds), Harrison's Principles of Internal Medicine, 18e.