<prompt> The Gold standard investigation for Gastro-esophageal reflux disease is:Endoscopy24 hour pH recordingOesophageal manometryMeasurement of length of lower oesophageal sphincter  <response>: b. 24 hour pH recordingRef: Nelsons 20e p1787-1788, Ghai 8e p 280Extended esophageal pH monitoring of distal esophagus, provides a quantitative sensitive documentation of acidic reflux episodes, the most important type of reflux episodes for pathologic reflux 