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: Medicine
To diagnose GERD , in 24 hour ambulatory pH testing the percentage of the day that the pH was less than 4 is: 
Select and explain. 
A. > 20%, B. > 1%, C. > 5 %, D. > 1 %
C: > 5 %
Answer: C: > 5 % 
Explanation: The percentage of the day that the pH was less than 4 with values exceeding 5% indicative of GERD.Intraluminal impedance monitoring can be added to pH monitoring to detect reflux events irrespective of whether or not they are acidic. Ref : Harrison, E-18, P- 2428