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
Percentage of patients with perforated peptic ulcer who show free gas under the diaphragm - 
Select and explain. 
A. 100%, B. 75%, C. 50%, D. 90%
B: 75%
Answer: B: 75% 
Explanation: • Upright chest x-ray shows free air in about 80% of patients of perforated peptic ulcer.