<prompt> Gold standard investigation for Gastro-esophageal reflux disease isLower esophageal pH monitoringLower esophageal pharyngeal pH monitoringEndoscopic examination of esophagusX-ray pharynx  <response>: Answer- A. Lower esophageal pH monitoring24 Hour pH monitoring for reflux esophagitis24 hour pH monitoring pH probe study, is the most sensitive test for diagnosis of gastroesophageal reflux. The test is performed by placing a catheter with a microelectrode that measures pH in the distal esophageal lumen for a duration of 24 hours. An episode of acid reflux usually is defined as an esophageal pH 4, for a duration greater than l5 - 20 seconds. 