This is a real-world medical entrance exam question, please give the true answer based on the question and selection. Topic name: None, Subject name: Surgery
Which of the following test differentiates between diffuse esophageal spasm and GERD 
Select and explain. 
A. Barium swallow, B. Manometry, C. Ambulatory 24 hour pH monitoring, D. Biopsy
C: Ambulatory 24 hour pH monitoring
Answer: C: Ambulatory 24 hour pH monitoring 
Explanation: Symptoms and manometric findings of diffuse esophageal spasm can be caused by GERD. To differentiate between them it is mandatory to do 24 hour pH monitoring.