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: Physiology
A patient got his SBP reduced by 10mmHg when he stood up and he gained only 8mmHg. What was the gain? 
Select and explain. 
A. –4, B. 4, C. –8, D. 8
A: –4
Answer: A: –4 
Explanation: Correction here is = –8mmHg
Error persists even after correction = 10 – 8 = +2
Gain = Correction / Error 
         = –8 / +2 
         = –4