This is a real-world medical entrance exam question, please give the true answer based on the question and selection. Topic name: Bleeding from the GUT and diseases of esophagus, Subject name: Medicine
Gold standard Investigation for Achalasia cardia is : 
Select and explain. 
A. Esophageal manometry, B. Barium swallow, C. Endoscopy, D. Endoscopic ultrasound
A: Esophageal manometry
Answer: A: Esophageal manometry 
Explanation: In long-standing achalasia, the esophagus may assume a sigmoid configuration. The diagnostic criteria for achalasia with esophageal manometry are impaired LES relaxation and absent peristalsis. High-resolution manometry has somewhat advanced this diagnosis; three subtypes of achalasia are differentiated based on the pattern of pressurization in the non peristaltic esophagus. Because manometry identifies early disease before esophageal dilatation and food retention, it is the most sensitive diagnostic test