This is a real-world medical entrance exam question, please give the true answer based on the question and selection. Topic name: Kidney and ureter, Subject name: Surgery
All are radiolucent stones except 
Select and explain. 
A. Triamterene stones, B. Uric acid stone, C. Xanthine stones, D. Cystine stones
D: Cystine stones
Answer: D: Cystine stones 
Explanation: Mnemonic - TIXU Triamterene Indinavir Xanthine Uric acid Radiolucent stones