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: Radiology
All of the following form radio–opaque stones except – 
Select and explain. 
A. Calcium dihydrate, B. Cysteine, C. Struvite, D. Uric acid
D: Uric acid
Answer: D: Uric acid 
Explanation: Cystine stones are radio-opaque.