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
All of the following are radio-opaque stones except: September 2009 
Select and explain. 
A. Calcium, B. Struvite, C. Uric acid, D. Cystine
C: Uric acid
Answer: C: Uric acid 
Explanation: Ans. C: Uric acid