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
Which of the following forms radiopaque stones: 
Select and explain. 
A. Xanthine., B. Oxalate., C. Uric acid., D. None.
B: Oxalate.
Answer: B: Oxalate. 
Explanation: Radiopaque stones: cysteine, oxalate & struvite
Radiolucent stones: xanthine, uric acid.
Allopurinol may lead to xanthine stones.